The hydrolysis of egg white by crystalline pepsin, trypsin, and chymotrypsin was inhibited by aniline, arginine, creatine, diethyldithiocarbamate, ethyl-enediamine, guanidine, lysine, o-phenylenediamine, pyro-phosphate, sulfanilic acid, and a variety of carbonyl group reagents. The hydrolysis of the same substrate by trypsin and chymotrypsin was also inhibited by N, N, N'', N'' -tetracarboxy-methylenediamine (Versene). The peptic hydrolysis of casein was inhibited by guanidine and by several carbonyl group reagents. The hydrolysis of casein by trypsin and chymo-trypsin was not inhibited (in contrast to the results with egg-white) by arginine, ethylenediamine, guanidine, lysine, and sulfanilic acid. The hydrolysis of carbobenzoxyglutamyl-tyrosine by pepsin was not significantly influenced by hydrazine, but was slightly (9-14%) inhibited by guanidine. The effect of trypsin on benzoylarginineamide was inhibited by arginine, guanidine, and lysine, but not by creatine, hydrazine, hydroxyl-amine, phenylhydrazine, and Versene. Dialysis and recrystalliza-tion expts. with enzyme-inhibitor mixtures led to the recovery of fully active enzymes and did not reveal the existence of a stable enzyme-inhibitor complex. The exptl. evidence suggested that not the enzymes but the substrates were influenced by the inhibitors listed above, in such a manner that they became more resistant to enzymatic attack.