Poly(ester-ether-ester) block copolymers, belonging to a class of biodegradable materials, were synthesized from poly(ethylene glycol) and ~-caprolactone by a simple ring-opening mechanism, which avoids the use of potentially toxic inorganic or organometallic initiators. The morphological and mechanical properties of such materials were investigated by gelpermeation chromatography, vapour pressure osmometry, proton magnetic resonance, infrared spectroscopy, differential scanning calorimetry, X-ray diffractometry and stress-strain tensile tests. The biocompatibility was investigated by cytotoxicity and hemocompatibility tests; the cytotoxicity was tested by the Neutral Red uptake assay, the 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyl tetrazolium bromide assay, the Kenacid Blue R-binding method, and by the cell proliferation test on polymer films; the hemocompatibility was tested by the contact activation both of the coagulation cascade (intrinsic pathway), by the plasma prekallikrein activation test, and of the thrombocytes, by measuring the release of platelet factor 4 and I~-thromboglobulin. The experimental results show that such a polymerization process permits high-molecular mass block copolymers with relatively good tensile and mechanical properties to be obtained. Their cyto-and hemo-compatibility makes them suitable for employment as biomaterials.