<PAGE>
 
                                                                   Exhibit 10.15

                         AGREEMENT AND PLAN OF MERGER

     THIS AGREEMENT AND PLAN OF MERGER (this "Agreement") is made and entered
into this 30th day of January, 1998, by and among NATIONAL ENVIRONMENTAL SERVICE
CO., an Oklahoma corporation ("NESCO"); LAB ONE ANALYTICAL, INC., an Oklahoma
corporation ("Lab One"); LAB ONE ACQUISITION, INC., an Oklahoma corporation and
a newly formed and wholly owned subsidiary of NESCO ("Acquisition Corporation");
EDDY L. PATTERSON, an individual residing in Tulsa, Oklahoma ("Patterson"); and
ALBERT A. MCCUTCHAN, an individual residing in Tulsa, Oklahoma ("McCutchan").

                                R E C I T A L S
                                ---------------

     WHEREAS, Patterson and McCutchan own in the aggregate all of the issued and
outstanding shares of common stock of Lab One; and

     WHEREAS, it is the desire of NESCO to acquire Lab One through a merger of
Lab One with NESCO's wholly-owned subsidiary, Acquisition Corporation, pursuant
to which Lab One would be the surviving corporation; and

     WHEREAS, it is the desire of Patterson and McCutchan that NESCO acquire Lab
One through such merger; and

     WHEREAS, in order to carry out the foregoing objectives, the parties hereto
desire to enter into this Agreement to merge Acquisition Corporation with and
into Lab One.

                               A G R E E M E N T
                               -----------------

     NOW, THEREFORE, in consideration of the mutual promises and agreements
hereinafter set forth, the parties agree as follows:

                        ARTICLE I.  TERMS OF THE MERGER.
                                    ------------------- 

     1.1. The Merger.  Pursuant to the terms and provisions of this Agreement
          ----------                                                         
and the Oklahoma General Corporation Act (the "Oklahoma Corporate Law"),
Acquisition Corporation shall merge with and into Lab One (the "Merger").

     1.2. Merging Corporations.  Acquisition Corporation shall be the merging
          --------------------                                               
corporation under the Merger and its corporate identity and existence, separate
and apart from Lab One, shall cease upon consummation of the Merger.

     1.3. Surviving Corporation.  Lab One  shall be the surviving corporation
          ---------------------                                              
(the "Surviving Corporation") in the Merger.  No change in the certificate of
incorporation, bylaws, directors, or officers of Lab One shall be effected by
the Merger.

     1.4. Effect of Merger.  The Merger shall have all of the effects provided
          ----------------                                                    
by the Oklahoma Corporate Law.  It is the understanding of the parties that the
Merger shall constitute a
<PAGE>
 
reorganization within the provisions of Section 368(a)(2)(E) of the Internal
Revenue Code of 1986, as amended (the "Code").

     1.5. Consideration.
          ------------- 

          a.  At the Effective Time (as hereinafter defined) each of the issued
     and outstanding shares of common stock of Lab One shall be converted into
     the right to receive 300 shares of NESCO Common Stock and $100.00 in cash
     (collectively, the "Consideration") for each share, with the aggregate
     consideration for all shares of common stock of Lab One to be 225,000
     shares of NESCO Common Stock and $75,000.00 cash.

          b.  All of the shares of common stock of Lab One, by virtue of the
     Merger and without any action on the part of the holders thereof, shall no
     longer be outstanding and shall be canceled and retired and shall cease to
     exist, and each holder of any certificate or certificates which,
     immediately prior to the Effective Time represented outstanding shares of
     Lab One (collectively, the "Certificates") shall cease to have any rights
     with respect to such shares, except the right of the holder to receive
     Consideration for each share upon the surrender of such Certificates in
     accordance with Section 1.7 below.
                     -----------       

          c.  Each share of common stock, par value $1.00 per share, of
     Acquisition Corporation outstanding immediately prior to the Effective Date
     shall convert into one share of Common Stock of the Surviving Corporation.

     1.6. Closing Date.  The closing of the transactions contemplated herein
          ------------                                                      
(the "Closing") shall take place on January 30, 1998, or on such other date as
the parties hereto may mutually agree (the "Closing Date").  The Merger shall be
effective upon the filing of a Certificate of Merger with the Secretary of State
of Oklahoma (the "Effective Time").

     1.7. Exchange Procedure: Surrender of Certificates.  At the Closing,
          ---------------------------------------------                  
Patterson and McCutchan shall deliver to NESCO the Certificates evidencing all
of the shares of common stock in Lab One that were converted into the right to
receive the Consideration in the Merger.  Upon surrender of such Certificates,
and all other documents that may be required by NESCO in connection therewith,
the holder of each such Certificate shall be entitled to receive, in exchange
therefor, the Consideration as provided in Section 1.5 above, with the portion
                                           -----------                        
of Consideration consisting of NESCO Common Stock represented by a certificate
or certificates issued by NESCO to the appropriate holder of such NESCO Common
Stock.

                   ARTICLE 2. REPRESENTATIONS AND WARRANTIES
                              ------------------------------

     2.1  Representations and Warranties by Patterson and McCutchan.  Patterson
          ---------------------------------------------------------            
and McCutchan each hereby represents and warrants to NESCO and Acquisition
Corporation that the shares of NESCO Common Stock he will receive in the Merger
are being acquired for his own separate account for investment only and not with
a view to, or for sale in connection with, any distribution of NESCO Common
Stock in violation of the Securities Act of 1933, as amended (the "Securities
Act"), or any rule or regulation under the Securities Act.  Patterson and
McCutchan each understand that (a) the NESCO Common Stock may not be transferred
or sold for value in the absence of registration or qualification or an
exemption from registration or qualification under the Securities Act, the
Oklahoma Securities Act, and the securities or Blue Sky laws of any other state,
<PAGE>
 
as may be required, (b) a stop transfer instruction will be issued with respect
to  NESCO Common Stock, and (c) the following legend will be placed on each
certificate representing NESCO Common Stock to be received in the Merger:

          The securities represented by this certificate have not been
          registered under the Securities Act of 1933, as amended, the Oklahoma
          Securities Act,  or any other state securities law. Neither the record
          nor beneficial ownership of said securities may be sold or transferred
          in the absence of an effective registration statement for such
          securities under said Acts unless, in the opinion of counsel
          satisfactory to the Company, exemptions from the registration
          requirements of said Acts are available with respect to such sales or
          transfer and such sale or transfer is made pursuant to and in strict
          compliance with the terms and conditions of said exemptions.

     2.2  Representations and Warranties by Lab One, Patterson, and McCutchan.
          -------------------------------------------------------------------  
Lab One, Patterson, and McCutchan, jointly and severally, hereby represent and
warrant to NESCO and Acquisition Corporation as follows:

          a.  Organization and Standing: Charter and Bylaws.  Lab One is a
              ---------------------------------------------               
     corporation duly organized and existing under, and by virtue of, the laws
     of the State of Oklahoma and is in good standing under such laws.  Lab One
     has the requisite corporate power to own and operate its properties and
     assets, and to carry on its businesses as presently conducted and as
     proposed to be conducted.  Lab One is qualified, licensed, or domesticated
     as a foreign corporation in any jurisdiction where the nature of its
     activities and of properties owned or leased by it makes such
     qualification,  licensing, or domestication necessary at this time. Lab One
     has furnished NESCO with copies of its respective certificate of
     incorporation and bylaws.  Said copies are true, correct, and complete and
     contain all amendments through the date of this Agreement.

          b.  Corporate Power and Authorization.  Lab One has all requisite
              ---------------------------------                            
     legal and corporate power to enter into this Agreement and to carry out and
     perform its obligations under the terms of this Agreement.  This Agreement
     is a legal, valid and binding obligation of each of Lab One, enforceable
     against each of Ward and Ward Services in accordance with its terms.

          c.  Capitalization.  All of the issued and outstanding shares of
              --------------                                              
     common stock of Lab One are, in the aggregate, held by Patterson and
     McCutchan.  The issued and outstanding shares of such common stock have
     been duly authorized and validly issued, are fully paid and nonassessable
     and were issued in compliance with all applicable state and federal laws
     concerning the issuance of securities. There are no outstanding rights,
     options, warrants, conversion rights, or agreements for the purchase or
     acquisition from Lab One of any shares of its capital stock.

          d.  Financial Statements and Other Information.  The financial
              ------------------------------------------                
     statements of Lab One provided by Lab One to NESCO, or to persons acting on
     NESCO's behalf, fairly present the financial condition and results of
     operations of Lab One as of the dates and for 
<PAGE>
 
     the periods thereof. Lab One has provided to NESCO all other material
     information regarding its business, operations, affairs and assets. None of
     the information provided to NESCO contained any material misstatements of
     fact and there were no omissions of facts which are necessary to make the
     information that was provided, in light of the circumstances in which the
     information was provided, not misleading.

     2.3. Representations and Warranties of NESCO and Acquisition Corporation.
          -------------------------------------------------------------------  
NESCO and Acquisition Corporation represent and warrant to Lab One, Patterson,
and McCutchan as follows:

          a.  Organization and Standing.  NESCO is a corporation duly organized
              -------------------------                                        
     and existing under, and by virtue of, the laws of the State of Oklahoma and
     is in good standing under its laws.

          b.  Organization and Standing.  Acquisition Corporation is a
              -------------------------                               
     corporation duly organized and existing under, and by virtue of, the laws
     of the State of Oklahoma and is in good standing under its laws.

          c.  Corporate Powers and Authorization.  Both NESCO and Acquisition
              ----------------------------------                             
     Corporation have all requisite legal and corporate power to enter into this
     Agreement and to carry out and perform their respective obligations under
     the terms of this Agreement. Neither the certificates of incorporation nor
     bylaws of NESCO or Acquisition Corporation, nor any other instrument to
     which NESCO or Acquisition Corporation are parties, or by which either is
     bound, nor any court order or any governmental law, rule or regulation,
     will be violated by execution and consummation of this Agreement by NESCO
     or Acquisition Corporation.  All corporate action on the part of NESCO and
     Acquisition Corporation, their directors, and stockholders necessary for
     the transactions contemplated by this Agreement have been taken.  This
     Agreement is a legal, valid, and binding obligation of NESCO and
     Acquisition Corporation, enforceable against each corporation in accordance
     with its terms.

          d.  Common Stock.  The NESCO Common Stock to be issued to Patterson
              ------------                                                   
     and McCutchan hereunder is duly authorized and, when issued in accordance
     with the terms of this Agreement, will be validly issued, fully paid, and
     nonassessable.



                        ARTICLE 3.  GENERAL PROVISIONS.
                                    ------------------ 

     3.1  Waivers and Amendments.  Any of the obligations or requirements set
          ----------------------                                             
forth in this Agreement may be waived, and any provision of this Agreement may
be amended, by written agreement of the parties hereto.

     3.2. Governing Law.  This Agreement shall be governed in all respects by
          -------------                                                      
the laws of the State of Oklahoma.

     3.3. Successors and Assigns.  Except as otherwise expressly provided
          ----------------------                                         
herein, the provisions hereof shall inure to the benefit of, and be binding
upon, the successors, heirs, executors,
<PAGE>
 
and administrators of the parties hereto. No party hereto shall assign this
Agreement, or any rights thereto, without the prior written consent of the other
parties hereto.

     3.4. Entire Agreement.  This Agreement and the other documents delivered
          ----------------                                                   
pursuant hereto constitute the full and entire understanding and agreement
between the parties hereto with regard to the subjects hereof and thereof.

     3.5. Notices.  All notices and other communications required or permitted
          --------                                                            
to be given to any party hereunder shall be in writing and shall be effective
when delivered at the address set forth below or at such other address as that
shall have furnished to the parties in writing:

          12331 East 60/th/ Street South
          Tulsa, Oklahoma 74145

     3.6. Severability.  In case any provision of this Agreement not material to
          ------------                                                          
the benefits intended to be conferred hereby shall be determined to be invalid,
illegal, or unenforceable, the validity, legality, and enforceability of the
remaining provisions shall not in any way be affected or impaired thereby.

     3.7  Survivability of Representations and Warranties.  The representations
          -----------------------------------------------                      
and warranties of the parties hereto shall survive the Closing of the Merger and
the other transactions contemplated hereby.

     3.8. Other Documents.  The parties to this Agreement shall in good faith
          ---------------                                                    
execute such other and further instruments, assignments or documents as may be
necessary or advisable to carry out the transactions contemplated by this
Agreement.

     3.9. Titles and Headings.  The titles of the Articles, Sections and
          -------------------                                           
subsections of this Agreement are for convenience of reference only and are not
to be considered in construing this Agreement.

     3.10.  Counterparts.  This Agreement may be executed in any number of
            ------------                                                  
counterparts, each of which shall be an original and all of which together shall
constitute one instrument, which instrument shall be deemed effective when there
exist copies signed by all of the parties hereto.

     IN WITNESS WHEREOF, the undersigned have caused this Agreement to be
executed by their duly authorized representatives, effective as of the date set
forth on the first page hereof.

"NESCO"                                   "LAB ONE"

National Environmental Service Co.,       Lab One Analytical, Inc.,
an Oklahoma corporation                   an Oklahoma corporation


By: /s/Larry G. Johnson                   By: /s/Eddy L. Patterson
   -----------------------------               ----------------------------    
   Larry Johnson,                              Eddy L. Patterson,
   Vice President                              Chairman of the Board
<PAGE>
 
"ACQUISITION CORPORATION"

Lab One Acquisition, Inc.,
an Oklahoma corporation


By: /s/Larry G. Johnson
    ----------------------------
    Larry Johnson,
    President

"PATTERSON"                                    "MCCUTCHAN"
 

   /s/ Eddy L. Patterson                       /s/ Albert A. McCutchan
   -----------------------------               ----------------------------    
   Eddy L. Patterson                           Albert A. McCutchan