SummarySoy and navy bean trypsin inhibitors considerably reduce the incidence of fatal trypsin embolism in rabbits with the retention of marked thromboplastic activity by the inhibitor-enzyme complex. The thromboplastic effect is observed with little or no proteolytic activity and the relative protection against embolism is apparent even with extensive proteolytic activity. Similar observations were made with the bean trypsin inhibitors and hypochlorite-treated trypsin, the latter having been shown earlier to have considerably less hypotensive effect than untreated trypsin. The differentiation of the soy and navy bean trypsin inhibitors is briefly considered and an improved method of preparing the navy bean inhibitor is given.