the authors developed two different methods to immobilize heparin on polymer surfaces. one method involves in situ heparin immobilization on a segmented polyurethane urea (biomer) surface via hydrophilic poly(ethylene oxide) (peo, mn $equals$ 4,000) spacers the other method uses peo/poly(dimethylsiloxane) (pdms) block co-polymer and heparin covalently linked in a block co-polymer system (peo-pdms-hep). these surfaces have demonstrated high heparin bioactivity in vitro and excellent blood compatibility in in vitro-ex vivo experiments. this report evaluates the long-term in vivo blood compatibility of these heparin immobilized surfaces. vascular grafts (6 mm id, 7 cm in length) were fabricated with biomer, and heparin was immobilized in situ with peo spacers (b-peo4k) and coated on their luminal surfaces with peo-pdms-hep. biomer and peo (mn $equals$ 4,000) grafted biomer (b-peo4k) were used as controls. the grafts were implanted in the abdominal aorta of dogs and retrieved at 3 months or when graft occlusion was suspected. retrieved grafts were evaluated with scanning electron microscopy (sem) and transmission electron microscopy (tem). (edited author abstract) 7 refs