<PAGE>

                                                                   EXHIBIT 10.30

                                 June 24, 1999



Mr. Ray Robbins, Individually and as President of
Robbins Enterprises, Inc.
2201 Ingleside
Grand Prairie, Texas 75050

RE:  Buyback of Training Bonus Agreement by Mannatech, Incorporated
Dear Mr. Robbins:

  This letter is to document the agreement between Mannatech, Incorporated
("Mannatech") and yourself (whether acting individually or on behalf of Robbins
Enterprises, Inc., a Texas corporation which you control) (collectively,
"Robbins") with regard to the repurchase of certain rights that you have to
payments from Mannatech under an agreement dated March 23, 1995 between Robbins
and Emprise International, now know as Mannatech ("March 1995 Agreement").

  Under the terms of the March 1995 Agreement, as stated in Paragraph 3(d)
thereof, you are entitled to receive training payments as specified therein
("Training Payments").  At the time that the March 1995 Agreement was entered
into between the parties, Mannatech was a fledgling, domestic company.
Mannatech, today, has grown significantly, operates in two foreign countries,
and contemplates further international expansion.  Robbins contends that the
March 1995 Agreement entitles him to Training Payments respecting international
business.  The Company has taken the position that the March 1995 Agreement
entitled Robbins only to business that was contemplated at the time of the
agreement, such being respecting domestic and Canadian business.  In order to
settle this difference of opinion, Mannatech has offered to repurchase of the
rights to Training Payments, whether based on foreign or domestic business, to
which Robbins has agreed ("Repurchase Agreement").  Robbins, hereafter in
corporate literature and presentations, will be referred to as a "founder" of
Mannatech.

  The total amount of the Purchase Price for the Repurchase Agreement shall be
the sum of $750,000.00 which shall be paid part in cash upon the execution of
this Letter Agreement, with the balance being paid by an earnout.  Upon the
execution of this Letter Agreement, Mannatech shall tender, in good funds, the
sum of $500,000.00 to Robbins as the Cash Portion of the Purchase Price  of the
Repurchase Agreement.  As of the date of the execution of this Letter Agreement
and

                                       1
<PAGE>

the receipt by him of the Cash Portion of the Purchase Price of the
Repurchase Agreement, Robbins shall no longer be entitled to payments under the
March 1995 Agreement, which shall in all respects be terminated as of that date.
Robbins shall, thereafter be entitled to the Earnout Portion of the Purchase
Price in an aggregate amount of $250,000.00, payable as follows:

  The Earnout Portion of the Purchase Price of the Repurchase Agreement shall be
$250,000.  On or before the thirtieth day of each month, commencing in July,
1999, and continuing on the same date of each subsequent month thereafter until
July, 2001 Mannatech shall pay the sum of $10,000 ("Monthly Payment Amount") to
Robbins.  Accordingly, Robbins shall receive and continue to receive the Monthly
Payment Amount until such time as the Earnout Portion of the Purchase Price
shall have been paid in full.  No interest shall accrue or be payable on the
Earnout Portion of the Purchase Price of the Repayment Agreement.

  Robbins and Mannatech have previously entered into an agreement to terminate
certain of the rights of Robbins, excluding the rights to receive Training
Payments.  This agreement, dated May 14, 1997 is attached hereto as Combined
Exhibit "A" ("May 1997 Agreement").  The May 1997 Agreement terminated all prior
agreements between the parties except that one covenant in the March 1995
Agreement requiring Mannatech to pay Training Payments to Robbins which remained
in effect.  Except as modified by this Letter Agreement, resulting in the
termination of Training Payments to Robbins by Mannatech, the May 1997 Agreement
shall remain in full force and effect and valid in every way.

  As a further inducement for Mannatech to enter into this Letter Agreement,
Robbins, on behalf of himself and Robbins Enterprises, Inc. represents and
warrants as follows:

  (a)  That Robbins is the sole owner of the right to receive Training Payments,
as currently continues to exist under the March 1995 Agreement, and that he has
not transferred, assigned, pledged or impaired in any his right to transfer
these rights back to Mannatech.

  (b)  That upon the execution of this Letter Agreement, which shall operate as
an Assignment to Mannatech, Mannatech shall have all and full right, title and
interest in and to the Robbins Training Payment rights to the effect that
neither he, nor any person, nor any entity shall have any right or claim of
right to receive such Training Payments.

  (c)  That there are, and will be at closing, no adverse or other claims to
receipt of or title to the Training Payments, except as made by Robert Hydeman
in his arbitration case against Robbins and Mannatech ("Hydeman Arbitration"),
which claims shall have been settled or shall be settled contemporaneously with
the execution of this Letter Agreement, nor will Robbins be upon notice of any
threat of the same. And otherwise, no party shall have been granted any form of
other right relating to receipt or right to receive the Training Payments. As of
the execution and delivery by Robbins of this Letter Agreement, there will not
be any action, suit or proceeding pending, or to the knowledge of the Robbins
threatened, against or affecting the Training Payments or any portion thereof.
All judicial proceedings concerning the Training Payments or any other dispute
arising

                                       2
<PAGE>

from, inter alia, the Hydeman Arbitration will be finally dismissed, and
terminated prior to or contemporaneously with the execution and delivery by
Robbins of this Letter Agreement.

  (d)  That Robbins is not currently in violation of or subject to any existing,
pending or threatened investigations or inquiry by any governmental authority or
to any remedial obligations under any applicable law.

  (e)  That from and after the date hereof, up to and including the date upon
which Robbins executes this Letter Agreement and receives the Cash Portion of
the Purchase Price of the Repurchase Agreement, which in all events shall be
prior to June 30, 1999, Robbins shall not sell, assign or create any right,
title or interest whatsoever in or to the Training Payments, create or permit to
exist any liens, encumbrances or charges thereon, without promptly discharging
the same.

  (f)   That Robbins shall properly pay, when due any tax liability resulting
from and due any taxing authority on account of the effect of this Letter
Agreement, and shall indemnify and hold Mannatech harmless from the same.
Robbins further represents and warrants that he is an independent contractor of
Mannatech for all purposes, including for federal and state income and other tax
purposes.

  All of the representations, warranties, covenants, agreements and indemnities
of Seller and Purchaser contained in this Agreement, to the extent not performed
at the Closing, shall survive the Closing and shall not be deemed to merge upon
the acceptance of the Deed of Purchaser.

  This Letter Agreement embodies and constitutes the entire understanding
between the parties with respect to the transactions contemplated herein, and
all prior or contemporaneous agreements, understandings, representations and
statements (oral or written) are merged into this Letter Agreement, except as
otherwise stated as to the survival of the effect of the May 1997 Agreement.
The foregoing notwithstanding, nothing in this Letter Agreement shall be deemed
to alter or amend any Associate Agreement of Robbins which creates an Associate
position in the Mannatech downline, the obligation of Mannatech to reimburse or
pay Robbins' reasonable travel expenses in connection with Mannatech business or
to permit him to designate as Presidential to permit him to designate as
Platinum Presidential (or the then highest rank of Mannatech Association
position) the Top Gun associate position (Associate Control Number 727),
reserving the right to designate at any later date the Duree/REI associate
position (Associate Control Number 6103) and the Auge/REI associate position
(Associate Control Number 994) as Platinum Presidential (or the then highest
rank of Mannatech Associate position), provided such designation will not
economically affect any Associate, rather only Mannatech.  None of the terms of
this Letter Agreement or any provision hereof may be waived, modified, amended,
discharged or terminated except by an instrument in writing signed by the party
against whom the enforcement of such waiver, modification, amendment, discharge
or termination is sought, and then only to the extent set forth in such
instrument.

  This Letter Agreement is for the sole benefit of Robbins and Mannatech, its
successors and

                                       3
<PAGE>

assigns, and no other person shall be deemed to have any privity of contract
hereunder, nor shall any other person be deemed to be a third party beneficiary
hereunder.

  This Agreement shall be governed by and construed in accordance with the laws
of the State of Texas.

  This Letter Agreement shall be binding upon and shall inure to the benefit of
the parties and their respective heirs, executors, administrators, legal
representatives, successors, and assigns.

  All references to the date of this Agreement or similar references shall be
deemed to refer to the date, in point of time, on which the Seller and Purchaser
have executed a copy of this Agreement, which shall be not later than June 30,
1999.

  With respect to the provisions of this Agreement, time is of the essence.
However, if the final date of any period which is set out in any provision of
this Agreement falls on a Saturday, Sunday or legal holiday under the laws of
the United States, the State of Illinois or the State of Texas, then the time of
such period shall be extended so that the last day of such period falls on the
next day which is not a Saturday, Sunday or legal holiday.

  The offer to repurchase the Training Payments upon the terms and conditions
set forth herein which is extended to Robbins by delivery of this Agreement to
him shall expire and be automatically revoked and cancelled unless he shall have
executed this Agreement and delivered at least one (1) copy to Seller on or
before 1:00 P.M. on June 30, 1999.


                                     Very truly yours,

                                     MANNATECH, INCORPORATED


                                     By:  /s/ Anthony E. Canale
                                          ----------------------------
                                     Its: Chief Operating Officer
                                          ----------------------------


Agreed as of the 30th of June, 1999


/s/ Ray Robbins
------------------------------------------
Ray Robbins, Individually and on behalf
or Robbins Enterprises, Inc., a Texas
Corporation.

                                       4
<PAGE>

                                   Exhibit A

                                   AGREEMENT

  This Agreement ("Agreement") is made and effective this 14th day of May, 1997,
by and between Mannatech, Incorporated ("Corporation"), a Texas corporation
whose principal place of business is 600 S. Royal Lane, Suite 200, Coppell,
Texas and Ray Robbins ("Consultant"), who resides at 2201 Ingleside, Grand
Prairie, TX  75050.

                                 WITNESSETH:

  WHEREAS, the Corporation is in the business of operating a network marketing
company which sells a proprietary line of dietary supplements, cosmetics and
over-the-counter drugs ("Products") and which compensates its distributors
("Associates") by a defined compensation plan;

  WHEREAS, in connection with the development of its business the Corporation
has entered into a confidential relationship with the Consultant whereby the
Consultant will acquire or has acquired an intimate knowledge of the
Corporation's business and will obtain or has obtained specialized skills.  The
Corporation will permit or has permitted the Consultant to have access to and to
utilize the business goodwill, cost and pricing information, CONFIDENTIAL
INFORMATION (as defined herein) and various trade secrets of the Corporation,
including without limitation, marketing programs, business relationships,
customer lists, and other compilations of information developed by the
Corporation and essential to its business;

  WHEREAS, the Consultant is a key to the business of the Corporation and the
Corporation will provide or has provided the Consultant with access to such
CONFIDENTIAL INFORMATION and trade secrets in reliance upon the Consultant
entering into this Agreement; and

  WHEREAS, in conjunction with the Consultant's access to and use of the
CONFIDENTIAL INFORMATION and trade secrets of the Corporation, the Consultant
has agreed to enter into this Agreement with the Corporation;

  WHEREAS, the Consultant and the Corporation have previously entered into a
series of Agreements, attached hereto as Compiled Exhibit "A" ("Prior
Agreements"), which the parties now desire to modify by this Agreement;

  NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein and upon the terms, conditions and provisions hereinafter set
forth, the Corporation and the Consultant do hereby agree as follows:

                                       5
<PAGE>

                                   ARTICLE I
                           CONFIDENTIAL INFORMATION

  1.  The Corporation will provide or has provided the Consultant with
specialized information concerning the products and the business operations of
the Corporation.  Irrespective of the term of consultancy, and in consideration
of the Consultant's promises specified in Article II of this Agreement, the
Corporation agrees to provide specialized training and instruction to the
Consultant for the duties in consultation to the Corporation concerning the
development of the Associate downline of the Corporation, recruitment and other
matters as may come before the parties from time-to-time, and agrees to provide
specialized training to the Consultant for such additional consulting duties as
the parties may in good faith agree in the future.

  2.  During the course of the Consultant's engagement and training incident
thereto the Consultant will be or was given access to the Corporation's
CONFIDENTIAL INFORMATION concerning Products and the business operations of the
Corporation.

  3.  The Consultant acknowledges that in the further course of the Consultant's
engagement with the Corporation, the Consultant will gain a close, personal and
special influence with the Corporation's customers and Associates and will be
acquainted with all of the Corporation's business, particularly the
Corporation's CONFIDENTIAL INFORMATION concerning the business of the
Corporation and its affiliates.

  4.  For purposes of this Agreement "CONFIDENTIAL INFORMATION" shall mean and
include information disclosed to the Consultant or known by the Consultant
through the Consultant's engagement with the Corporation, not generally known in
the Corporation's industry, about the Corporation's products, processes and
services, including but not limited to information concerning inventions, trade
secrets, research and development, as well as all data or information concerning
customers (including, Associates), customer lists (including downline reports
and similar reports of business activities and relevant information concerning
persons who conduct the same), prospect lists, mailing lists, sales leads,
contracts, financial reports, sales, purchasing, price lists, product costs,
marketing programs, marketing plans, business relationships, business methods,
accounts payable, accounts receivable, accounting procedures, control procedures
and training materials.

  5.  The Consultant recognizes that the Consultant's position with the
Corporation is one of the highest trust and confidence by reason of the
Consultant's access to the CONFIDENTIAL INFORMATION and the Consultant agrees to
use the Consultant's best efforts and will exercise utmost diligence to protect
and safeguard the CONFIDENTIAL INFORMATION.  In this respect, the Consultant
agrees that fulfilling the obligations of the Agreement is part of the
Consultant's job responsibilities with the Corporation for which the Consultant
has been retained as an Consultant and for which the Consultant has received
consideration therefor.

                                       6
<PAGE>

  6.  Except as may be required by the Corporation in connection with and during
the Consultant's engagement with the Corporation, or with the express written
permission of the Corporation, the Consultant shall not, either during the
Consultant's work as an consultant with the Corporation or at any time
thereafter, directly or indirectly, download, printout, copy, remove from the
premises of the Corporation, use for the Consultant's own benefit or for the
benefit of another, or disclose to another, any CONFIDENTIAL INFORMATION of the
Corporation, its customers, contractors or of other with which the Corporation
has a business relationship.

  7.  Consultant agrees that all files, memoranda, data, notes, records,
drawings, charts, graphs, analyses, letters, reports, or other documents or
similar items made or compiled by the Consultant, made available to the
Consultant or otherwise coming into the Consultant's possession while engaged by
the Corporation concerning any process, apparatus or products manufactured,
sold, used, developed, investigated or considered by the Corporation concerning
the CONFIDENTIAL INFORMATION or concerning any other business or activity of the
Corporation shall remain at all times the property of the Corporation and shall
be delivered to the Corporation upon termination of the Consultant's engagement
with the Corporation or at any other time upon request.

  8.  The Consultant agrees that, during the term of the Consultant's engagement
with the Corporation or upon termination thereof, and if requested by the
Corporation to do so, the Consultant will sign an appropriate list of any and
all CONFIDENTIAL INFORMATION  of the Corporation of which the Consultant has
knowledge or about which the Consultant has acquired information.

                                  ARTICLE II
                           DUTIES AND NONCOMPETITION

  1.   Consultant's duties shall be those assigned agreed to between the
Corporation from time-to-time, but shall include development of recruiting and
Associate downline matters.  Nothing contained in this agreement is intended to
abrogate or affect the right of the Consultant to be paid in accordance with the
compensation plan of the Corporation, as the same if from time-to-time in
effect.  The Consultant agrees that the Consultant will serve the Corporation
diligently, faithfully and to the best of the Consultant's ability during the
term of the Consultant's engagement with the Corporation, which shall be at the
will of the Corporation, and which shall further be subject to the announced
Policies and Procedures of the Corporation which shall from time-to-time be in
effect.  The Consultant shall devote reasonable time, efforts, ability and
attention to the business of the Corporation and the performance of the
Consultant's duties, understanding that the Consultant is an independent
contractor for all purposes, including Federal Income Tax purposes.

  2.  Consultant acknowledges and understands that from time to time the
Consultant's duties will require the Consultant to work at non-company
locations.  In such instance, the

                                       7
<PAGE>

Consultant agrees to comply with all of the policies, procedures and directives
relevant to working at such non-company locations.

  3.  Consultant represents and admits that in the termination of the
Consultant's engagement for any reason whatsoever, the Consultant's experiences
and capabilities are such that the Consultant can obtain engagement in business
engaged in other lines and/or of a different nature, and that the enforcement of
a remedy by way of injunction will not prevent the Consultant from earning a
livelihood.

  4.  Consultant acknowledges that the Consultant will receive special knowledge
and specialized training from the Corporation, included in which is the
CONFIDENTIAL INFORMATION identified in Article I above.  The Consultant further
acknowledges that training provided by the Corporation and the CONFIDENTIAL
INFORMATION is valuable to the Corporation and, therefore, the Corporation's
investment in the training and the protection and maintenance of the
CONFIDENTIAL INFORMATION constitutes a legitimate interest to be protected by
the Corporation by the covenant not to compete, set forth in Paragraph 5 of
Article II of this Agreement.

  5.  The Consultant therefore agrees that during the term of the Consultant's
engagement with the Corporation and for a period of one (1) year (or a shorter
period with the written consent of the Corporation) after the Consultant shall
cease to be engaged by the Corporation for any reason, the Consultant shall not,
directly or indirectly engage in or solicit, as an owner, principal, director,
officer, partner, consultant, engaged, distributor, agent of any business which
competes with that of the Corporation in the United States of America.
Additionally, the Consultant agrees that he/she shall not, without the prior
written consent of the Corporation, for a period of one (1) year from the
termination of the Consultant's engagement with the Corporation (or a shorter
period with the written consent of the Corporation) in the United States of
America:

      a.  Solicit any current or prospective customer, Associate, partner,
principal or investor of the Corporation;

      b.  Directly or indirectly be engaged by, invest in, manage or engage in
any business that is competitive with the business of the Corporation;

      c.  Either as an individual, a partner, or a joint venturer, or in any
other capacity whatsoever, directly or indirectly accept engagement with or
render services to any business that markets products and services in
competition with those marketed by or under development by the Corporation at
the time of the termination of the Consultant's engagement with the company or
offers its goods and/or services for sale via a sales mechanism such as multi-
level marketing, network marketing or direct marketing.

  As set forth above, the Consultant acknowledges that this noncompletion
covenant is

                                       8
<PAGE>

ancillary to or a part of an otherwise enforceable agreement, such being the
general agreement of Engagement and its related agreements concerning
confidentiality and non-disclosure of CONFIDENTIAL INFORMATION and non-
solicitation, at the time that this noncompetiton covenant is made, that the
limitations as to time defined herein are reasonable and do not impose a greater
restraint than is necessary to protect the goodwill or other business interests
of the Corporation, that the limitations as to geographic area defined herein
are reasonable and do not impose a greater restraint than is necessary to
protect the goodwill or other business interests of the Corporation, and that
the scope of activity to be restrained defined herein is reasonable and does not
impose a greater restraint than is necessary to protect the good will or other
business interests.

  6.   The Consultant agrees that during his engagement with the Corporation and
for a period of one (1) year after termination of such engagement, he/she shall
not, on his/her own behalf or on behalf of any other person or business entity,
hire solicit, seek to hire or offer engagement to any person who now or later
works for the Corporation, or who is a current or prospective employee of the
Corporation.  The Consultant further agrees that he will not in any other manner
attempt, directly or indirectly, to influence, entice or encourage any person
who now or later works for the Corporation, or who is a current or prospective
employee of the Corporation, to leave the engagement of the Corporation during
the operative term of this covenant of Non-Solicitation:

  a.  Any corporation, partnership, association, agency, or business, engaged in
the business of multi-level marketing, network marketing and/or direct selling
or in the marketing, sales and distribution of dietary and nutritional
supplement products in competition with the Corporation or any of its affiliates
at the time the Consultant ceased to be engaged by the Corporation; or

  b.  Any corporation, partnership, association, agency, or business, known to
the Consultant to be an Associate or customer of the Corporation as of the date
of the Consultant's cessation of engagement by the Corporation or with whom the
Consultant had contact during, and by reason of, the Consultant's engagement by
the Corporation.


  7.  Consultant agrees that in the highly competitive business in which the
Corporation is engaged, personal contact is of primary importance in securing
new and retaining present Associates and customers.  The Consultant also agrees
that the Corporation has a legitimate interest in maintaining its relationships
with its Associates and customers and that it would be unfair for the Consultant
to solicit the business of the Corporation's Associates and customers and
exploit the personal relationships the Consultant develops with the
Corporation's Associates and customers by virtue of the Consultant's access to
the Corporation's customers as a result of the Consultant's engagement by the
Corporation.

  8.  The Consultant therefore agrees that for a period of one (1) year after
the

                                       9
<PAGE>

Consultant shall cease to be engaged by the Corporation for any reason, the
Consultant shall not solicit, in competition with the Corporation, the business
of any person, company, firm, corporation, partnership, association, agency, or
business, known to the Consultant to be an Associate and/or customer of the
Corporation as of the date of the Consultant's cessation of engagement by the
Corporation or with whom the Consultant had contact during, and by reason of,
the Consultant's engagement by the Corporation.

  8.  The Consultant further agrees that for a period of one (1) year after the
Consultant shall cease to be engaged by the Corporation for any reason, the
Consultant will not, either directly or indirectly, through any person, firm,
association or corporation with which the Consultant is now or may hereafter
become associated, cause, influence or induce any present or future employee of
the Corporation or its affiliates to leave the employ of the Corporation or its
affiliates to accept engagement with the Consultant or with such person, firm,
association, or corporation with whom the Consultant may then be affiliated.

  10.  The foregoing covenants not to compete shall not be held invalid or
unenforceable because of the scope or the territory or actions subject thereto
or restricted thereby, or the period of time within which such Agreement is
operative; but award or decree in arbitration or any judgement of a court of
competent jurisdiction, as the case may be, may define the maximum territory and
actions subject thereto and restricted by this Article II and the period of time
during which the Agreement is enforceable.  Any alleged breach of other
provisions of this Agreement asserted by the Consultant shall not be a defense
for the Consultant to claims arising from the Corporation's enforcement of the
provisions of this paragraph.  Should the Consultant violate the noncompletion
covenants of this Article II, then the period of time for these covenants shall
automatically be extended for the period of time from which the Consultant began
such violation until the Consultant permanently ceases such violation.

  11.  Irrespective of the term of engagement under this Agreement, and in
consideration of the promises specified in Article II of this Agreement, the
Corporation agrees as follows:

  a.  To issue, within thirty days of the date of this Agreement, capital stock
of the Corporation in an amount of 939,333 shares, it being agreed that
notwithstanding any provision contained herein to the contrary, the Consultant
agrees to be bound by the underwriting agreements or requirements by and between
the Corporation and any Underwriter which might provide services to it in
connection with any public offering of its capital stock ("Underwriter").
Further, should such Underwriter impose any restrictions upon the exercise,
registration or other rights, concerning the of stock conferred hereby or which
is otherwise owned by the Consultant, the Consultant agrees to further be bound
by such requirements, limitations, restrictions, and/or agreements, as agreed to
by the Corporation.  You hereby appoint Mannatech (the Corporation) as your
attorney-in-fact to execute all documents on your behalf concerning agreements
offering the shares of stock which are conferred hereby, including, without
limitation, those agreements with the Underwriter, referenced above; which the
parties agree is of a value in excess of $1,000.00.

                                       10
<PAGE>

  b.  To provide specialized training as specified herein;

  c.  To provide the Consultant with access to the Corporation's software and
files, records, marketing procedures, processes, computer programs, compilations
of information, records, Associate and client requirements, pricing techniques,
lists, formulae, lists identifying Associates, partners, potential investors,
methods of doing business and other CONFIDENTIAL INFORMATION which is regularly
used in the operation of the business of the Corporation as is relevant to the
Consultant's engagement in the opinion of the Corporation.

  12.  The Consultant agrees that the agreements, noncompetition agreements,
nondisclosure agreements, and non-solicitation agreements set forth herein each
constitute separate agreements, independently supported by good and adequate
consideration and shall be severable from the other provisions of this Agreement
and shall survive the Agreement.  The existence of any claim or cause of action
of the Consultant against the Corporation, whether predicated on this agreement
or otherwise, shall not constitute a defense to the enforcement by the
Corporation of the covenants and agreements of the Consultant contained in the
noncompetiton, nondisclosure or the non-solicitation agreements.  If a court of
competent jurisdiction determines that any restriction in a clause or provision
of this Agreement is void, illegal or unenforceable, the other clauses and
provisions of this Agreement shall remain in full force and effect and the
clauses and provisions that are determined to void, illegal or unenforceable
shall be limited so that they shall remain in effect to the fullest extent
permitted by law.

  13.  The Consultant will use his best efforts to ensure that no relative of
his, nor any corporation or other entity or which he is a officer, principal,
manager, director or shareholder or other affiliate, shall take any action that
the Consultant could not take without violating any provision of this Agreement.

  14.  The Consultant recognizes and acknowledges that damages in the event of
his breach of certain provisions of this Engagement Agreement would be
inadequate, and the Consultant agrees that the Corporation, in addition to all
other remedies it may have, shall have the right to injunctive relief via
arbitration if there is a breach by the Consultant of any one or more of the
provisions contained in Article II hereof.

  Arbitration shall be the exclusive remedy for any and all disputes, claims or
controversies, whether statutory, contractual or otherwise, between the
Corporation and the Consultant concerning the Consultant's engagement or the
termination thereof.  In the event either party provides a Notice of Arbitration
of Dispute to the other party, the Corporation and the Consultant agree to
submit such dispute or controversy, whether statutory or otherwise, to an
arbitrator selected from a panel of arbitrators of the American Arbitration
Association located in Dallas, Texas.  The effective rules of Commercial
Arbitration of the American Arbitration Association shall control the
Arbitration. In any arbitration proceeding conducted subject to these
provisions, all statutes of limitations that would otherwise be applicable shall
apply to any arbitration proceeding hereunder.  In any arbitration proceeding
conducted subject to these

                                       11
<PAGE>

provisions, the arbitrator is specifically empowered to decided any question
pertaining to limitations, and may do so by documents or by a hearing, in his or
her sole discretion. In this regard, the arbitrator may authorize the submission
of pre-hearing motions similar to a motion to dismiss or for summary
adjudication for the purposes of consideration this matter. The arbitrator's
decision will be final and binding upon the parties. The parties further agree
to abide by and perform any award rendered by the arbitrator. The prevailing
party in such proceeding shall be entitled to record and have awarded its
reasonable attorney's fees, in addition to any other relief to which it may be
entitled. In rendering the award, the arbitrator shall state the reasons
therefor, including any computations of actual damages or offsets, if
applicable.

                                  ARTICLE III
                           ASSIGNMENT OF INVENTIONS

  1.  The Consultant agrees to promptly disclose to the Corporation and
Consultant hereby assigns to the Corporation or its designee, its assigns,
successors or legal representatives, all, right, title and interest in and to
any and all patents, formulae, inventions, processes, designs, software,
firmware, circuitry, diagrams, copyrights, trade secrets, and any other
proprietary information (collectively, the "Proprietary Information")
whatsoever, conceived, developed or completed by the Consultant during the
course of the Consultant's engagement with the Corporation, or using the
Corporation's time, data, facilities and/or materials, provided the subject
matter of the Proprietary Information is within the scope of the duties and
responsibilities of one in the Consultant's position with the Corporation or
occurs as a result of the Consultant's knowledge of a particular interest of the
Corporation.

  2.  The Consultant agrees to assist the Corporation at any time during the
Consultant's engagement with the Corporation, or after termination of the
Consultant's engagement by the Corporation with reimbursement by the Corporation
for all expenses incurred, in the preparation, execution, and delivery of any
assignments, disclosures, patent applications, or papers within the scope and
intent of this Agreement required to obtain patents or copyrights in the
Proprietary Information in this or a foreign country and in connection with such
other proceedings as may be necessary to vet title to the Proprietary
Information in the Corporation, its assigns, successors, or legal
representatives.

                                  ARTICLE IV
                                 MISCELLANEOUS

  1.  Termination.  Nothing contained in this Agreement shall be construed as
impairing the right of the Corporation to terminate the Consultant's engagement
with the Corporation hereunder.

                                       12
<PAGE>

  2.  Obligations.  The Consultant's obligations under this Agreement shall
continue, survive and remain enforceable in accordance with the terms hereof,
whether or not the Consultant's engagement with the Corporation shall be
terminated voluntarily or involuntarily, with or without reason.

  3.  Enforcement.  It is the express intention of the parties to this Agreement
to comply with all laws applicable to the covenants and provisions contained in
this Agreement.  If any of the covenants contained in this Agreement are found
to exceed in duration or scope those permitted by law, it is expressly agreed
that such covenant may be reformed or modified by the, an award or decree of an
arbitrator, or, if applicable, a final judgement of a court of competent
jurisdiction or other lawful constituted authority, as the case may be, to
reflect a lawful and enforceable duration or scope, and such covenant
automatically shall be deemed to amended and modified so as to comply with the
arbitration award, decree, judgment or order of such court or authority, as the
case may be.  If any one or more of the provisions contained herein shall for
any reason be held invalid, illegal or unenforceable in any respect even after
reformation, such invalidity, illegality or unenforceability shall not affect
the enforceability or validity of any other provision contained in this
Agreement, and this Agreement shall be construed as if such invalid, illegal, or
unenforceable provisions had never been contained herein.

  4.  Waiver of Breach.  The waiver by any party hereto of a breach of any
provision of this Agreement shall not operate or be construed as a waiver of any
subsequent breach by any party.

  5.  Entire Agreement.  This Agreement contains the entire agreement of the
parties hereto.  no modification or amendment of this Agreement may be made
except by written agreement signed by both of the parties hereto.

  6.  Descriptive Headings.  All headings, captions and arrangements used in
this Agreement are intended solely for the convenience of the parties and shall
not be deemed to limit, amplify or modify the terms of this Agreement nor affect
the meaning thereof.

  7.  Governing Law.  The substantive laws of the State of Texas, excluding any
conflicts of law rule or principle that might otherwise refer to the substantive
law of another jurisdiction, shall govern the interpretation, validity and
effect of this Agreement without regard to the place for performance thereof.
This Agreement has been executed and delivered by the parties hereto in Dallas
County, Texas, and the Corporation and the Consultant agree that venue as to any
action which might ensue after arbitration shall be proper with the state of
federal courts in Dallas County, Texas to decide any matter relating to this
Agreement.

  8.  Notices.  Any notice or communication required or permitted hereby shall
be in writing and shall be delivered personally, sent by prepaid telegram and
followed with a confirming letter, or mailed by certified or registered mail,
postage prepaid.

                                       13
<PAGE>

  (a)  If to the Consultant, to:
       Ray Robbins
       2201 Ingleside
       Grand Prairie, TX  75050

  (b)  If to the Corporation, to:

       Mannatech, Incorporated
       600 S. Royal Lane, Suite 200
       Coppell, Texas 75019

   or in the case of each party hereto, to such other address and to the
   attention of such other person as may have theretofore been specified in
   writing in like manner by such party to the other party. Each such notice or
   communication shall be deemed to have been given as of the date so delivered
   or at the expiration of the third business day following the date of the
   mailing.

  9.  Assignment.  This Agreement shall insure to the benefit of and be binding
upon the Corporation and the Consultant and their respective successors and
assigns.  The Consultant shall not be entitled to assign any rights or
obligations hereunder.

  10.  Prior Agreement.  This Agreement supersedes all prior agreements set
forth in Compiled Exhibit "A"  between the parties of any and every nature
whatsoever, including agreements for additional compensation or benefits.   All
such prior agreements are null and void.  Robbins is not, shall not be, and has
not been a Vice President of the Corporation.  He shall not, hereafter be
designated as a "National Director of Sales and Marketing"; however the
Corporation shall seek to determine a designation which is appropriate to his
independent contractor status which is reasonably agreeable to the Consultant.
The foregoing not with standing nothing contained in this provision shall be
construed as to alter any compensation due to the Consultant as an Associate
under the compensation plan of the Corporation, as the same is from time-to-time
in effect, for the operation of his Mannatech business, or to in any way alter
his agreement for compensation regarding the training of Associates which has
been in effect since the inception of the business of the Corporation.

  11.  Consultant Acknowledgement.  The Consultant affirms and attests by
signing this Agreement that Consultant has read this Agreement before signing it
and that the Consultant fully understands its purposes, terms, and provisions,
which the Consultant hereby expressly acknowledges to be reasonable in all
respects.  The Consultant further acknowledges receipt of one (1) copy of this
Agreement.

                                       14
<PAGE>

  IN WITNESS WHEREOF, the parties hereto have signed this Agreement as of
this19th day of August, 1997, and effective as of May 14, 1997.

                                       Consultant:

                                       /s/ Ray Robbins
                                       ----------------------------
                                       Ray Robbins



                                       Other entities to be bound:
                                       (List)

                                       -----------------------------

                                       -----------------------------

                                       -----------------------------

                                       -----------------------------

                                       -----------------------------

                                       -----------------------------

                                 ACKNOWLEDGEMENT

THE STATE OF TEXAS    (S)
                      (S)
COUNTY OF DALLAS      (S)

  BEFORE ME, the undersigned Notary Public, personally appeared RAY ROBBINS, and
known to me to be the person whose name is subscribed to the foregoing
instrument, and acknowledged to me that he executed the above instrument for the
purposes and consideration expressed therein.

  GIVEN UNDER MY HAND AND SEAL OF OFFICE on this 19th day of August, 1997.



                                       /s/ Cheryl Anderson
                                       -----------------------------------
                                       Notary Public, State of Texas

                                       15