Polyurethane surfaces were modified by covalent immobilization of different molecular weight heparins (fractionated Mw similar to 3000 Da, and unfractionated Mw similar to 17,000-19,000 Da) and examined by ESCA, AFM, and contact angle goniometer. The effect of these different surface-immobilized heparins on blood protein adsorption and on platelet adhesion, were examined after incubating the samples with platelet-poor and platelet-rich plasma. Protein adsorption kinetics was studied by electrophoresis and the platelets adhered on the surfaces were examined by SEM after incubation. The two heparin types clearly showed different behavior with respect to protein adsorption, especially in the early stages of blood plasma interaction. After a 5-min incubation in plasma, low molecular weight heparin-immobilized polyurethanes (PU-LMWH) showed three times less protein adsorption compared to unfractionated heparin-immobilized polyurethanes (PU-UFH). The total amount of adhered protein became more similar as the incubation time was extended. The morphology of adhered platelets on material surfaces demonstrated differences: PU-UFH had clusters with some pseudopodia extensions, while PU-LMWH surfaces had round-shaped platelets with little clustering. For contact times > 15 min, the amount of adsorbed proteins and adhered platelets on the heparinized PU surfaces decreased.