We studied the effect on both platelet aggregation and blood coagulation, known to be major risk factors in thrombogenesis, of proteins from hen egg yolk (EP). EP potently inhibited collagen-induced human platelet aggregation in a dose-dependent manner. Furthermore, EP has a synergistic effect on the inhibition of human platelet aggregation with both molsidomine, an inhibitor of cGMP-specific phosphodiesterase, and theophylline, an inhibitor of cAMP-specific phosphodiesterase. These results indicate that the active mode of EP might be involved in elevation of the levels of both cGMP and cAMP. Prothrombin time and activated partial thromboplastin time were potently prolonged by EP. These data suggest that EP prolongs the time interval between the conversion of fibrinogen to fibrin. Accordingly, these findings demonstrate that EP might have antithrombotic effects by inhibiting platelet aggregation and fibrin formation.