Studies were performed on Dacron vascular prostheses (USA), Gore-Tex (Germany), polyester prostheses (Czechoslovakia), Lawsan (Russia), as well as on DALLON-standard and DALLON-double velour prostheses (Poland), non-impregnated or impregnated with collagen, with heparin and albumin, with antithrombotic and antibacterial preparation, and with antibacterial preparation. In vitro, all the materials of vascular prostheses cause an adhesion of platelets in different degree, increase the availability of platelet factor 3, release platelet factor 4, reduce the blood clot retraction, shorten clotting time of the whole blood as well as recalcination time of intact plasma and increase the prothrombin consumption. None of the materials induced platelet aggregation nor influenced the activity of fibrinolytic system. Prostheses impregnated with collagen or with heparin and albumin were the most thrombogenic. Those impregnated with antibacterial preparation or with antibacterial and antithrombotic preparations, and Gore-Tex prostheses showed the lowest thrombogenicity. Thrombogenicity of vascular prostheses not only depends on chemical structure of the material, but on the method of fibre manufacturing by individual producers as well.