Summary. The presence of trypsin inhibitors in human seminal plasma was demonstrated, and their effect on the acrosomal trypsin-like enzyme (TLE), as well as on other proteolytic enzymes, was studied. Two inhibitor fractions with approximate molecular weights of 11,500 and 4000 were obtained by gel filtration. The small molecular fraction could only inhibit the activity of trypsin but the larger one completely in- hibited the activity of chymotrypsin and partially inhibited that of tissue kallikrein. Neither of the fractions could inhibit the activity of plasmin, thrombin or urokinase. Both inhibitors were heat stable. The activity of acrosomal TLE was absent in ejaculated intact human and bull spermatozoa, but it could be evoked by repeated washings of the spermatozoa with an electrolyte solution. Both inhibitor fractions inhibited the TLE-activity of intact spermatozoa as well as that of an acrosomal preparation. One of the inhibitors was effective only when dialysed acrosomal preparation or washed spermatozoa in low con- centrations were used as the source of the enzyme.