Pellethane@ 2363 60Afilms and tubings were chemically modified and the effect of these modifications on platelet deposition was studied. Grafting of high molecular weight poly(ethylene oxide) and graft polymerization of methoxy poly(ethylene glycol) 400 methacrylate resulted in surfaces with a good water wettability. The increased hydrophilicity of these modified surfaces could be demonstrated by contact angle measurements. The platelet deposition was investigated with tubings in a capillary flow system, using different types of perfusates. Platelet deposition from a buffer-containing perfusate on surfaces modified with either high molecular weight poly(ethylene oxide) or methoxy poly(ethylene glycol) 400 methacrylate was almost absent and less than on Pellethane 2363 60A. Using a titrated plasmacontaining perfusate the amount of deposited platelets on Pellethane 2363 60A modified with high molecular weight poly(ethylene oxide) was low and about the same as on unmodified surfaces. However, a marked reduced platelet deposition compared to unmodified Pellethane 2363 60A was found when the platelets were activated by Ca2+ ionophore. The improved blood compatibility of the modified Pellethane 2363 60A tubings obviously indicates the favourable effect of the presence of grafted PEG on the surface. Keywords: Copolyether urethanes. poly(ethylene oxide), grafting, Platelet depositionPoly(ethylene oxide) (PEO) is more and more regarded as a polymer with interesting blood contacting properties. The low affinity of PEO for proteins and other blood components has stimulated many investigators to study the interactions of blood and biomaterials based on PEOle7.In an attempt to improve the blood compatibility of a commercial copolyether urethane, Pellethane@ 2363 80A (Pell80A). several techniques for grafting PEO onto Pell80A were investigated. Grafting of high molecular weight PEO with dicumyl peroxide (DCP) and graft polymerization of methoxy poly(ethylene glycol) 400 methacrylate (M PEGMA-400) were examined. The first method is based on crosslinking of high molecular weight polyethers. Cross-linked blends of poly(propylene oxide) and PEO have shown a good blood compatibility4. Surface analysis of these blends suggested that the good blood contacting properties of these materials may be ascribed to preferential presence of PEO at the polymer-water interface'. In the present work, Pell 80A substrates were dipped in a solution of PEO and DCP. After drying, the PEO/DCP coated substrates were UV or heat treated in order to form a network of PEO and Pell