A low molecular weight antiplasmin has been detected in human blood platelets. The antiplasmin is dialysable and a similar material is present in normal plasma. A simple method for rapid isolation of the antiplasmin based on ultrafiltration is described. The inhibition of plasmin by these materials under different conditions has been studied. In the ultracentrifuge, both antiplasmins showed a single broad peak with an approximate sedimentation coefficient of 0.5S. Paper chromatography and paper electrophoresis indicated the isolated material to be heterogeneous. The individual components were isolated by paper chromatography and the antiplasmin activity was measured by the fibrin plate method. Preliminary studies on the fraction with maximal antiplasmin activity suggest that the inhibitory effect might be due to enzyme-inhibitor complex formation. Based on the present data, it is concluded that the platelet antiplasmin and the serum antiplasmin are very similar. This antiplasmin material may be useful in fibrinolytic therapy.