FEARNLEY'S method for estimating fibrinolytic activity of normal blood1 is based on the fact that the clot formed by adding thrombin to blood diluted 1 : 10 with phosphate buffer will lyse on incubation. This lytic activity may be lost if the specimen is permitted to stand for hours or even minutes at room temperature. Thus Fearnley and others caution that specimens must be kept in an ice bath and preferably collected in pre-chilled vessels. In contrast, the techniques used in our studies of fibrinolysis induced by nicotinic acid2 employing essentially undiluted plasma and the ‘thrombelastograph’3 fail to show spontaneous lysis of normal plasma. The lytic activity induced by parenterally administered nicotinic acid is clearly demonstrable by our methods, and is not destroyed by many hours storage at room temperature or even two weeks at 5° C.