The method developed to measure transient thrombus deposition from arterial blood on polymeric surfaces involves infusion of small amounts of 125I-labeled fibrinogen and 51Cr-labeled platelets into the circulation of anesthetized mongrel dogs. Radioactivity adsorbed to the walls of femoral A-V shunts was counted to measure fibrin and platelet deposition with time following initial blood-polymer contact. Shunt segments were obtained at each datum for scanning electron microscopy of adhered thrombus material. Plasticized polyvinyl chloride (PVC), recognized as having poor blood compatibility, elicited rapid platelet and fibrin activation resulting in a transient maximum of thrombus deposition after 10 to 15 minutes of blood contact. Adherent thrombi subsequently left the surface. Contact of blood with segmented polyetherurethane (BIOMER) resulted in minimal platelet and fibrin deposition. Morphological analysis by electron microscopy confirmed these observations. This method can be used to determine the thrombogenic and embolic potential of polymeric surfaces and may be used to elucidate the mechanisms of artificial surface induced thrombosis.