Studies on Compatibility between Plastics and BloodSummary. Studying the compatibility of plastics and blood, at the one hand analyses were undertaken on artificial hearts implanted in calves, at the other hand in vitro techniques were gained by working for the purpose of testing several (different) fabrics simultaneously. Concerning the in vivo examinations there were made observations and documentations using light microscopy of the plastics' surfaces. Concerning the i~ vitro-methods there were tested blood and plasma samples in drops and in cavities of the following plastic materials with the special respect of their moisteness, expansion and their relation to the surfaces during longer incubation: Silastic, Silastic combined with Dacron-Velour, Silastic with DacronFlock, Silastic with graphite, Teflon, Hydron as well as PVC. At least, several fabrics were tested after having seeded them with cultivated fibroblasts looking at the ability of forming "endothelinm-like membranes" at the surfaces of those plastics.As to the techniques used in these experiments there were taken light microscopic methods m as told --, as well as phase contrast and stereomicroscopie techniques, moreover there were used immunologic and tissue culture methods.The findings of artificial hearts implanted, indicated layers of fibrin and thrombi in endo-and pericardial areas, especially in the part of atrium and in the region of vascular origins of the right heart. --In the first in vitro series, Teflon showed the best repelling effect against blood and plasma. --In the second in vitro row a deposit of fibrinogen (and fibrin), in some cases also of albumin and immunoglobulins on the plastics' surfaces and a diminution of fibrinogen in the plasma samples incubated on plastics could be seen. m In the experiments with fibroblasts in contact with fabrics' surfaces, the smooth Silastic preparation proved to be the best one for stimulating an endothelial layer.