In our country, FOY[Etryl-p-(6-guanidinohexanoyloxy )benzoate methanesulfonate], a newly developed protease Inhibitor, has been used in the treatment of DIC with promising result It has been postulated that FOY may possess certain effect en platelet function, besides its proved action on coagulation and fibrinolysis system. Therefore, in vitro effect or FOY on platelet aggregation was Investigated, utilizing aggregometer and human PRP.When PRP was preincubated with FOY UOQug/ml in final concentration), the secondary aggregation by ADP (4μg/ml) was completely Inhibited. However, FOY did not exhibit any inhibitory effect m primary aggregation Induced by lower concentration of ADP even with higher amount of FOY (up to 362μg/mn). FOY completely Inhibited aggregation Induced by thrombin (0.5 U/ml), when it was preincubated with PRP for 2 minutes. 30 μg/ml of FOY caused disaggregation when it was added to the mixture or PRP and thrcmbln immedlately after aggregation was Initiated. The Dremixture of thrombin and FOY did not cause aggregation of PRP. 10μg/ml of FOY completely inhibited aggregation induced by 0.7 mN of arachldonate, when It was preincubated with PRP for 2 minutes. Also, 100μg/ml of FOY exhibited similar Inhibitory effect on aggregation induced by collagen. It was not surprising for FOY to Inhibit platelet aggregation induced by thrombin, since it was round to possess competitive inhibition arainst thrcefcin. However, it is interesting for FOY to Inhibit the secondary platelet agfreration by ADP and by arachldonate.