Proteases Rcand Kcof the nuclei of rat testis cells exhibit considerable differences in susceptibility to various protease inhibitors. Protease Rcis inhibited by D‐phenylalanyl‐L‐propyl‐L‐arginine chlormethylketone and p‐nitrophenyl‐p1‐guanidino ben‐zoate. Protease Kcis inhibited by Nα‐p‐tosyl‐L‐lysine chlormethylketone, N‐p‐tosyl‐phenylalanine chlormethylketone, N‐carbobenzoxy‐L‐phenylalanine chloromethylketone and p‐nitrophenyl‐p'‐guanidino‐benzoate. Neither protease is inhibited by 10 mM phenylmethanesulphonyl‐fluoride or p‐chloromercuri‐benzoate. p‐nitrophenyl‐p1‐guanidino‐benzoate is a substrate for protease Rcwith release of p‐nitrophenol, however the protease activity is not restored during a period of 8 hours after removal of excess p‐nitrophenyl‐p1‐guanidino‐benzoate. Both proteases can bind to immobilized soybean trypsin inhibitors.