In lysates of washed human platelets produced by sonication or by addition of nonionic detergent, fibrinogen (Mr 340,000) was rapidly degraded, under conditions favorable to activation of the endogenous calcium-activated protease (CAP), to a core derivative (Mr 280-290,000) composed of partially degraded A alpha chains (Mr 47,000, 46,000, and 34,000) and B beta chains (Mr 56,000), and apparently intact gamma chains (Mr 53-54,000). Extensive degradation occurred within one minute at 4 degrees C, ambient temperature or at 37 degrees C, and was inhibited by leupeptin, EDTA, EGTA, or N-Ethylmaleimide, but not by soybean trypsin inhibitor, hirudin, aprotonin, benzamidine, phenylmethylsulfonyl fluoride or epsilon-aminocaproic acid. Purified plasma fibrinogen exposed to lysates containing active protease was cleaved in an identical fashion. The cleavage pattern of A alpha chains produced by this platelet protease activity is different from that produced by plasmin in vitro or that found in fibrinogen catabolites in vivo, and is unlike that produced by any cellular fibrinolytic enzyme yet described. In view of this finding, as well as the striking differential inhibitory effect of the agents cited above, we conclude that the degradation of platelet fibrinogen observed in these studies is due to direct proteolysis by platelet CAP.