The covalent attachment of activated polyethylene glycol2 (PEG2) of 10,000 daltons to non- essential groups on a serine proteinase II (SepII) from Aspergillus sojae produced two modified preparations (PEG2-SepII-S and PEG2-SepII-L). The molecular weights of PEG2-SepII-S and PEG2-SepII-L were about 170,000 and 280,000, respectively. The PEG2-SepII-S lost about 80 % of its antigenicity, while the PEG2-SepU-L completely lost its antigenicity. In comparison of kinetic parameters with SepII there was less than 40 % variation in Km, but the values of kcat towards succinyl-l-leucy 1-l-leucy 1-l-valy 1-l-tyrosine 4-methylcoumaryl-7-amide (Suc-LLVY-MCA) or succinyl-l- alanyl-l-alanyl-l-valyl-l-alanine β-nitroanilide (Suc-AAVA-/>NA) decreased to about 70% less than that of SepII. The modified preparations have about 20 % activity towards fibrin hydrolysis and a low affinity for a protein proteinase inhibitor, Streptomyces subtilisin inhibitor (SSI), with a molecular weight of 23,000, while the preparations have hig...