Effects of proteinase inhibitors on total proteolytic activity and trypsin and chymotrypsin activity in human pancreatic juice were determined separately. Purified inhibitors as well as crude extracts of raw soybeans completely inhibited trypsin and chymotrypsin activity while 40 to 50% of the total proteolytic activity remained. Inhibition experiments with 1,10-o-phenanthroline showed that this residual proteolytic activity was due mainly to carboxypeptidase A and B. Comparative studies with rat pancreatic enzymes demonstrated certain similarities between the corresponding enzymes from rat and man. However, differences were revealed which indicate that the rat enzymes must be used with great caution when applied as models for the human proteinases when studying effects of soybean inhibitors.