Subtilisin has been modified with polyethylene glycol (PEG) monomethacrylate (MW 8000) by reductive alkylation, and incorporated into polymethyl methacrylate during free-radical initiated polymerization, The activity and stability of the PEG-modified enzymes have been determined in aqueous buffer and organic solvents. The K-m and V-max values for unmodified, singly and doubly modified subtilisin were compared in these environments, and the half-lives of both modified enzymes were remarkably high (up to 2 months). The protein-containing polymer was analyzed for activity and polymer properties, and our results indicate that active subtilisin can be incorporated into polymethyl methacrylate during polymerization in organic solvents while retaining its activity and stability.