The purpose of the study was to investigate the in vitro inhibition of the proteolytic activity in samples of duodenal juice obtained during instillation of raw soybeans in humans. The results suggested the presence of an inhibitor-resistant trypsin. Practically no inhibition was obtained with phenylmethylsulfonyl fluoride, aprotinin, human alpha 1-antitrypsin or lima bean trypsin inhibitor. Specificity against synthetic substrates was different from that obtained in the absence of raw soybeans. Duodenal aspirates containing this new trypsin activity were able to further inhibit tryptic and chymotryptic activity of basal aspirates from the duodenum, demonstrating surplus amounts of inhibitors in the duodenal juice. Further incubation of trypsin-inhibitor complexes at 37 degrees C, in the presence of surplus uninhibited (trypsin and chymotrypsin) duodenal juice, restored tryptic activity. This activity was not inhibitor-resistant. The results obtained for trypsin activity based on esterase assays were confirmed by a proteolytic assay. It is concluded that the increase in tryptic activity in human duodenal juice during raw soybean infusion is due to a previously unidentified inhibitor-resistant trypsin and not due to inactivation of the inhibitors in raw soybeans.