Four studies were performed to evaluate the thrombogenicity of different central venous catheter materials. Two of these studies consisted of evaluating the amount of platelet deposition on different catheter materials, firstly in vitro and then in vivo using dogs. In these studies, 51-chromium was used to label the platelets. In the following study, the volume of clot and the degree of fibrin sheath were determined by placing catheters in both arteries and veins of dogs for two to four weeks before removing the vessels and performing a quantitative analysis. For the fourth study, indium labelled platelet deposition with scintillation counting was performed on six dogs in whom catheters had been placed in the femoral and carotid arteries. The vessels in three of these animals were removed 48 hours after imaging was completed to correlate the scintigraphic findings with a quantitative analysis of the clot and fibrin sheath on each catheter. There was a high degree of correlation between all these studies. The most consistently thrombogenic catheter material was polyurethane, and the least thrombogenic catheter material was polyurethane coated with hydromer. Silicone was the next least thrombogenic material examined.