Porcine trypsin obtained from pancreas residues subsequent to insulin removal undergoes autolysis when subjected to chromatography and gives rise to new forms of autolyzed products with intra-chain split at bonds Lys145-Ala146 and Arg105-Val106. Incubation of 1% solutions of porcine trypsin either at pH 5.0 or at pH 9.1 induces autolysis to give active products involving one or two specific cleavages of bonds Lys145-Ala146 and Arg105-Val106 or Lys131-Ser132, as well as inactive degraded products. No evidence has been obtained that on autolysis of porcine trypsin, and active fragment with molecular weight lower than that of the parent molecule was identified. The active forms of autolyzed products of porcine trypsin have almost the same specific activity as the intact enzyme when assayed against BAEE. They are of the same molecular weight as the parent molecule. These findings indicate that that active forms of autolyzed products maintain the specific three-dimensional structure essential for the catalytic activity of the trypsin molecule.