<PAGE>

                                                                    EXHIBIT 10.6

                  SIXTH AMENDMENT TO LEASE AGREEMENT BETWEEN
                AETNA LIFE INSURANCE COMPANY, AS LANDLORD, AND
                        LUMINEX CORPORATION, AS TENANT

               To be attached to and form a part of Lease made
               the 1st day of August, 1989 (which together with
               any amendments, modifications and extensions
               thereof, is hereinafter called the Lease), between
               Landlord and Tenant, covering a total of 32,859
               square feet and located at 12212 Technology
               Boulevard, Suites I, J and K and 12112 Technology
               Boulevard, Suite 200, Austin, Texas, known as
               McNeil #4 and McNeil #5, respectively.

WITNESSETH that the Lease is hereby amended as follows:

     1.   WHEREAS, Tenant needs additional space for its business purposes and
Landlord has available an area nearby effective on the Commencement Date, the
demised premises shall contain, in addition to the approximately 32,859 square
feet originally demised ("original space"), an additional area, hereinafter
called the "new space", containing approximately 18,330 square feet located at
12201 Technology Boulevard, Suite 130 (known as McNeil #3) (see Exhibit "A")
attached hereto, thus making the aggregate area of the premises approximately
51,189 square feet.  The term for the new space (Suite 130 at McNeil #3) shall
end thirty-six (36) months after the Commencement Date (as defined below), and
the Term for the existing space shall end on March 31, 2002 as provided in the
Lease.

The Commencement Date for the new space is defined as thirty (30) days following
existing tenant's vacation of the "new space" and the completion of
responsibilities outlined in Paragraph 3 and Paragraph 13 of this document.  The
Commencement Date is contemplated to be June 1, 2000.

     2.   The Monthly Rental shall be as follows:

<TABLE>
<CAPTION>
---------------------------------------------------------------------------------------------------------------
                           Monthly Base
                            Rental PSF       Monthly Base    Monthly Base     Monthly Base
     Term                 Existing Space        Rental       PSF New Space     Rental New      Total Monthly
                           (32,859 SF)      Existing Space    (18,330 SF)         Space         Base Rental
---------------------------------------------------------------------------------------------------------------
<S>                       <C>               <C>              <C>              <C>              <C>
Commencement -3/31/02          $0.75        $24,644.25       $0.80            $14,664.00       $39,308.25
---------------------------------------------------------------------------------------------------------------
  4/01/02 - 5/31/03              N/A               N/A       $0.80            $14,664.00       $14,664.00
---------------------------------------------------------------------------------------------------------------
</TABLE>

These amounts shall be in addition to property taxes, common area maintenance,
management fees, and insurance as provided in the Lease.

     3.   Previous tenant or Landlord shall be responsible for servicing and
repairing, if necessary, the HVAC system by a mutually acceptable, licensed
contractor and cleaning the "new space" to broom clean condition, including but
not limited to, removal of trash and previous tenant's equipment.

     4.   Landlord shall be responsible for exterior ADA compliance at the new
 space.

     5.   Landlord shall provide a tenant finish allowance of $54,990.00 to be
applied toward interior improvements.  Tenant shall bear the entire cost of any
interior improvements to be installed by Landlord in the premises in excess of
the finish-out allowance of $54,990.00 and shall pay for such excess over the
allowance as hereinafter provided.  In no event shall credit be given to Tenant
for any allowance not utilized. All improvements must comply with Trammell Crow
Company's standard specifications (see Standards and Specifications for
                                       --------------------------------
Office/Warehouse Buildings) and all applicable governmental regulations.  Prior
--------------------------
to beginning construction of any such improvements, Tenant shall submit
architectural drawings of the proposed improvements to Landlord and shall obtain
Landlord's written consent to begin construction.  Notwithstanding any provision
herein to the contrary, Tenant will utilize a general contractor, approved by
Landlord, to construct the interior improvements in the new space.  Tenant will
have the right to access, occupy and conduct business in the new space up to,
but not exceeding, thirty (30) days prior to the
<PAGE>

Commencement Date without any rent obligation. Tenant will construct the
interior improvements in the new space with reasonable diligence, but need not
finish them prior to the Commencement Date; such improvements may be constructed
while Tenant is occupying and conducting business in the new space. Tenant may
draw the allowance from Landlord from time to time as construction bills come
due.

     6.   Tenant shall have the right and option to renew its Lease relative to
the "new space", 12201 Technology Boulevard, Suite 130 (McNeil 3) for one (1)
additional thirty-six (36) month term by delivering written notice thereof to
Landlord at least One Hundred Eighty (180) days prior to the expiration date of
the lease term, provided that at the time of such notice and at the end of the
lease term, Tenant is not in default hereunder.  Upon the delivery of said
notice and subject to the conditions set forth in the preceding sentence, this
Lease shall be extended upon the same terms, covenants and conditions as
provided in this Lease, except that the rental payable during said extended term
shall be the prevailing market rental rate for space of comparable size, quality
and location at the commencement of such extended term (the "Market Rate"). This
right and option shall apply to the initial lease term only.  In the event
Landlord and Tenant are unable to agree upon the Market Rate, Landlord and
Tenant shall each promptly appoint a real estate broker who is licensed by the
Texas Real Estate Commission (TREC) and active in the Austin industrial market,
to assist in the determination of the Market Rate, and the two brokers shall
appoint a third broker who is also licensed by the Texas Real Estate Commission
and active in the Austin industrial market.  The determination of the Market
Rate by the agreement of any two of such three brokers shall be accepted by and
shall be binding upon Landlord and Tenant as the Market Rate, which rate shall
thereafter be payable until further adjusted.  Landlord and Tenant agree to use
all reasonable diligence to cause their appointed brokers to perform in good
faith and in a timely manner in order to make the determination of the Market
Rate on or before the date on which the Market Rate is to become effective.  In
the event the brokers do not make such determination in a timely manner, this
Lease shall nevertheless continue in full force and effect until such
determination is made, and the rental for such period shall be payable at the
rate otherwise payable hereunder.  Upon the determination of the Market Rate,
the payment of the Market Rate shall commence on the first day of the month
following the date of such determination, and in addition to such monthly
installment of rental, Tenant shall pay to Landlord the increase in rental
payable hereunder, if any, applicable to the period from the date on which the
Market Rate was scheduled to become effective to the payment of the first
installment at the Market Rate.  Landlord and Tenant shall each bear the cost
and fees of their respective brokers and shall share equally the cost of the
third broker, if needed.

     7.   Tenant, at its own cost and expense, shall enter into a regularly
scheduled preventive maintenance/service contract with a maintenance contractor
approved by Landlord for servicing all hot water, heating and air conditioning
systems and equipment within the Premises.  The service contract must include
the replacement of filters on a regular basis and all services suggested by the
equipment manufacturer in its operations/maintenance manual and must become
effective within thirty (30) days of the date Tenant takes possession of the
Premises.  Provided that Tenant maintains such service contract and is not in
default under the terms of this Lease, Landlord shall be responsible for any
repairs or replacements necessary to maintain the HVAC equipment in the new
space for a period of ninety (90) days following the Commencement Date.

     8.   Tenant shall have the nonexclusive right to utilize no more than
fifty-five (55) parking spaces at the new space.

     9.   Tenant's "proportionate share" of McNeil #3 is 26.8%.

     10.  Subject to Landlord's written approval of the installation process and
procedure, Tenant, at its expense, shall have the right to install
telecommunication lines connecting the new space and original space.

     11.  Tenant shall not use, and shall not permit any subtenant, licensee,
concessionaire, employee, agent or invitee (hereinafter collectively "Tenant's
Representatives") to use, any portion of the Premises or Building, for the
placement, storage, manufacture, disposal or handling of any hazardous materials
(hereinafter defined) unless Tenant complies with all applicable environmental
laws (federal,
<PAGE>

state or local), including, but not limited to those for obtaining proper
permits. In the event Tenant or Tenant's Representatives desire to use or place
hazardous materials on the Premises it shall notify Landlord in writing thirty
(30) days prior to such proposed use or placement, and provide the names of the
hazardous materials, procedures to insure compliance with the applicable
environmental law and such other information as Landlord may reasonably request.

     In the event Tenant or Tenant's Representatives places, releases or
discovers any hazardous materials on the Premises or Building in violation of
applicable environmental laws, Tenant shall immediately notify Landlord of such
fact in writing within twenty-four (24) hours of the placement, release or
discovery.  Tenant shall not attempt any removal, abatement or remediation of
those hazardous materials on the Premises in violation of applicable
environmental laws, without obtaining the additional written consent of
Landlord, which consent may be specifically conditioned on Landlord's right to
approve the scope, timing and techniques of any such work and the appointment of
all contractors, engineers, inspectors and consultants in connection with any
such work.  Tenant shall be responsible for the cost of any removal, abatement
and remediation work of any hazardous materials placed, stored, manufactured,
disposed of or handled by Tenant or Tenant's Representatives on the Premises or
any other portion of the Building and for the cost of any removal, abatement or
remediation of any hazardous materials which might be disturbed or released as a
result of any remodeling or construction in the Premises by Tenant or Tenant's
Representatives.  Such costs shall include, without limitation, the cost of any
supervision by Landlord, its employees or agents, in connection with such work.
Tenant shall comply with all environmental laws in connection with any such
removal.

     Tenant shall indemnify Landlord, its shareholders, directors, officers,
employees and agents and hold them harmless, from and against any loss, damage
(including, without limitation, a loss in value of the Building or damages due
to restrictions on marketing contaminated space), cost, liability or expense
(including reasonable attorneys' fees and expenses and court costs) arising out
of the placement, storage, use, manufacture, disposal, handling, removal,
abatement or remediation of any hazardous materials by Tenant or Tenant's
Representatives on the Premises or Building, or any removal, abatement or
remediation of any hazardous materials required hereunder to be performed or
paid for by Tenant, with respect to any portion of the Premises or the Building,
or arising out of any breach by Tenant of its obligations under this paragraph.

     The term "hazardous materials" as used herein shall mean (i) any "hazardous
waste" as defined by the Resource Conservation and Recovery Act of 1976 (42
U.S.C. Section 6901 et seq.), as amended from time to time, and regulations
promulgated thereunder; (ii) any "hazardous substance" as defined by the
Comprehensive Environmental Response, Compensation and Liability Act of 1980 (42
U.S.C. Section 9601, et seq.), as amended from time to time, and regulations
promulgated thereunder; (iii) asbestos or polychlorinated biphenyls; (iv) any
substance the presence of which on the Building or on the Premises is prohibited
or regulated by any federal, state or local law, regulation, code or rule; and
(v) any other substance which requires special handling or notification of any
federal, state or local governmental entity in its collection, storage,
treatment, or disposal.

     12.  This Lease shall be subordinate to any deed of trust, mortgage, or
other security instrument (a "Mortgage"), or any ground lease, master lease, or
primary lease (a "Primary Lease"), that now or hereafter covers all or any part
of the Premises (the mortgagee under any Mortgage or the lessor under any
Primary Lease is referred to herein as "Landlord's Mortgagee"), including any
modifications, renewals or extensions of such Mortgage or Primary Lease,
provided that it is a condition to the subordination of this Lease that the
Landlord's Mortgagee execute and deliver to Tenant a non-disturbance and
attornment agreement, in recordable form, whereby such Landlord's Mortgagee
agrees that neither this Lease nor Tenant's right to possession will be
terminated so long as Tenant is not in default under this Lease (provided
Landlord's Mortgagee, or successor thereto, maintains its rights to terminate
under other provisions of the Lease, e.g., for a casualty).  Notwithstanding the
foregoing, Tenant agrees that any such Landlord's Mortgagee shall have the right
at any time to subordinate such Mortgage or Primary Lease to this Lease on such
terms and subject to such conditions as Landlord's Mortgagee may deem
appropriate in its discretion.  Tenant agrees upon demand to execute such
further instruments subordinating this Lease or attorning to the Landlord's
Mortgagee as Landlord may request,
<PAGE>

provided such contains non-disturbance provisions as noted above and such other
provisions which do not affect the substantive provisions of this Lease. In the
event that Tenant should fail to execute any subordination or other agreement
required by this paragraph, within twenty (20) days of written request, it shall
constitute an Event of Default, without further notice required of Landlord.

     13.  Landlord, at Landlord's expense, will repair all roof penetrations and
service and repair all existing overhead doors.

Except as herein and hereby modified and amended the Agreement of Lease shall
remain in full force and effect and all the terms, provisions, covenants and
conditions thereof are hereby ratified and confirmed.


DATED AS OF THE 25th DAY OF APRIL, 2000.

WITNESS:                            AETNA LIFE INSURANCE COMPANY:

                                    By:  UBS Brinson Realty Investors LLC,
                                         Its Investment Advisor and Agent

/s/ Belinda Hanson                  /s/ S. Mark Cypert
-------------------------           ----------------------------------------
                                    By: S. Mark Cypert
                                    ----------------------------------------
                                    Title: Director
                                    ----------------------------------------


WITNESS:                            LUMINEX CORPORATION:

/s/ John M. Dapper                  /s/ James L. Persky
------------------------            ----------------------------------------
                                    By: James L. Persky
                                    ----------------------------------------
                                    Title: Vice President & Chief
                                             Financial Officer
                                    ----------------------------------------