A method to prevent the loss of enzymatic activity of proteolytic enzymes toward high molecular weight substrates that occurs upon derivatiza tion with monomethoxypoly(ethylene glycol) (mPEG) is described. It is based on the heterogenous phase enzyme modification after the enzyme is bound to an active site inhibitor immobilized on an insoluble resin. This procedure pro tects the active site and the surrounding area from mPEG linkage. Trypsin modified by mPEG in a heterogeneous phase, using benzamidine bound to Sepharose maintained a high degree of its ability to hydrolize large molecular weight substrates, such as bovine serum albumin or casein, compared to the mPEG derivatives obtained without any protection or with free benzamidine in solution.