Conjugates of F(ab')2 fragment of the monoclonal antibody A5B7 coupled to carboxypeptidase G2 (CPG2) have been produced using the heterobifunctional reagents 2-mercapto-[S-acetyl]acetic acid, N-hydroxysuccinimide ester (SATA) and m-maleimidobenzoyl-N-hydroxysuccinimide ester (SMPB). The effect of various levels of modifying reagent on enzyme activity and antigen binding activity were determined, and it was shown that whilst CPG2 is relatively sensitive to modification, insertion of three maleimide groups per CPG2 resulted in the loss of 30% of enzyme activity; A5B7 F(ab')2 was insensitive to modification, little or no activity being lost. The coupling efficiency of the reaction was shown to be fairly constant over a wide range of substitution levels. There was thus no advantage to be gained in using high substitution levels, which may result in loss of enzyme activity. The formation of undesired high molecular weight aggregates could be controlled by adjustment of the protein concentration during the final coupling step.