In vitro degradation and release of five types of heparin/surfactant-loaded poly(D,L-lactide-co-glycolide 50:50) (PLG) microspheres alone and also incorporated within microporous polyurethane tubes were studied over a 3-month period. Degradation was studied with scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Heparin release was characterized using a modified Azure A assay. SEM suggests that microspheres may be entrapped within polyurethane fibrils of the polyurethane tubes, thereby reducing contact with their hydrated environment. FTIR transmittance spectra confirm microsphere incorporation within the polyurethane tubes and PLG ester hydrolysis occurring over the 3-month period. A correlation was observed between decreasing molecular weights and glass transition temperatures (Tg). The microspheres alone exhibited a change in Tg but not when incorporated within the microporous tubes. Release profiles revealed a burst effect occurring during the first 4h and total release of the heparin from the microspheres by 12 weeks.