An increase in the lysis of human and rat clot of euglobulin fraction of blood plasma was caused by an actomyosin preparation isolated from rabbit and rat muscle. Neither fibrinolytic nor activating properties of actomyosin were observed on applying of different concentration (13-0.6 mg/ml) of the protein on the fibrin films. No activation of 0.4% solution of pure plasminogen by actomyosin was observed in vitro. A decrease in fibrin clot density under the effect of actomyosin was indicated by thromboelastogramms of blood plasma and euglobulins. The phenomenon was probably determined by the physicochemical properties of the protein itself.