A synthesis of the N-hydroxysuccinimide ester of S-acetylthioacetic acid is described. This material is stable when stored dry and has advantages over the currently available reagents used to introduce sulfhydryl groups into a variety of proteins. Proteins modified with this reagent can be used to prepare conjugates for enzyme immunoassay. The conjugation techniques described cause little or no loss of either enzyme activity or antibody titer and function, and the conjugates contain little polymeric material.