Trypsin-specific and trypsin-plasmin inhibitors were isolated from seminal vesicles of guinea pigs. Two different procedures were used: 1. Inhibitor material obtained from perchloric acid extracts was purified by affinity chromatography (using water insoluble trypsin resin) and gradient elution chromatography on Sulfoethyl Sephadex. Mainly two very similar trypsin-specific inhibitors and five somewhat different trypsin-plasmin inhibitors were obtained. (The amino acid compositions are given in Table 3). 2. Also by avoiding the trypsin resin step several inhibitor fractions were obtained which were differing considerably in their amino acid compositions. Inhibitors containing a lysine residue in the reactive site are reversibly inactivated by acylation with maleic anhydride; arginine inhibitors are inactivated by reaction with a butandion-2,3 reagent. In the reactive site of the trypsin-specific inhibitor the sequence Arg-Ile is present. The modified inhibitor (Argile bond is broken) is inactivated by incubation with carboxypeptidase B or reaction with excessive maleic anhydride. The native inhibitor (Arg-Ile bond intact) is converted into the modified form both during contact with the trypsin resin and by incubation with 2.3 mole percent trypsin. From acidic extracts of boar seminal plasma a trypsinplasmin inhibitor was isolated by affinity chromato-This article contains parts of E. FINK, Dissertation,