The effects of plasmin treatment upon washed human platelets were studied in an attempt to elucidate the mechanisms underlying thrombin-induced platelet aggregation. At calcium concentrations of 10–20 μM, plasmin (0.2 CTA U/ml) inhibited thrombin-induced aggregation almost completely, but did not diminish the thrombin-induced release of adenine nucleotides, 5-hydroxytryptamine, or calcium. Increasing the calcium concentration partially antagonized plasmin’s inhibition of aggregation. Studies utilizing calcium chelators and the Kunitz soybean trypsin inhibitor (SBTI) as a plasmin inhibitor indicated that in order to achieve maximal block of aggregation, plasmin must act upon a substrate made fully available only after an initial thrombin-platelet interaction has taken place. Moreover, the time course of this inhibition parallels the time course of the thrombin-induced release reaction. Plasmin inhibition of aggregation could not be mimicked by exposing the platelets to proteolytic digests of fibrinogen at concentrations as high as 17% total platelet protein. Nor could inhibitory activity be recovered from supernatants of plasmin -treated platelets, upon centrifugation and treatment with SBTI. With the use of a “cold initiation” technique, the release by thrombin of 46.7 ± 6-7 (mean ± SEM) μg of fibrinogen immunological equivalents per mg platelet protein could be demonstrated. Platelets in which thrombin-induced aggregation was abolished by plasmin treatment (and the plasmin subsequently inactivated by SBTI) aggregated normally upon addition of as little as 10 μg human plasma fibrinogen per mg platelet protein. It is concluded that plasmin inhibition of aggregation most likely results from its attack upon a protein that is released or becomes fully available subsequent to interaction of thrombin with a platelet receptor mediating release. The results of this study are consistent with a cofactor role for fibrinogen in the aggregation of human platelets by thrombin. * From a thesis submitted by Dr. J. L. Miller in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Connecticut. * This work was presented in part at the 57th Annual Meeting of the Federation of American Societies for Experimental Biology, Atlantic City, N. J., April 1973 (Federation Proceedings 32, 289).