The sulfhydryl protease B was isolated from the cotyledons of 8-day old vetch seedlings and purified 1580-fold with a 38% recovery. The preparation obtained proved to be homogeneous by DEAE-cellulose chromatography and polyacrylamide gel electrophoresis. The molecular weight of the enzyme as shown by Na-DS gel electrophoresis is 38 000. Protease B hydrolyzes the peptide bonds involving the carboxyl groups of asparagine in insulin chains A and B. Since protease B fails to attack the native reserve proteins the high molecular weight products of the initial hydrolysis of reserve proteins by the earlier discovered protease A seem to be the most probable substrates of protease B. A considerable part of these substrates is split by protease B to form large peptides. The role of protease B in reserve proteins degradation is discussed.