The products of plasma clot destruction by the low-frequency ultrasound (US) were analyzed by the combination of SDS gel-electrophoresis, gel filtration chromatography and scanning electron microscopy. It was found that US (27 kHz) did not cause activation of the plasmin system or covalent bonds cleavage in the fibrin molecules. With the US intensities less than 21.6 W/cm2 the extraction of blood serum proteins, which are located in the pores of the fibrin network was occurred. An increase of the intensity of ultrasonic action led to protofibril dissociation, which was accompanied by further release into the solution of the blood serum proteins located inside the fibrin fibers. Being extracted from aggregated plasma clot proteins free protofibrils formed insoluble fibrin particles.