In vascular catheterizations, the most serious complications are thrombotic and embolic accidents. Thereforemany experimental and clinical studies have been made on thrombogenicity of angiographic catheters, but there aredifficulties in observing or obtaining the thrombi formed around catheters. Owing to recent advance inradioimmunoassay, we can evaluate platelet activation and fibrin formation indirectly by measuring plasma plateletfactor 4(PF4), beta-thromboglobulin(BTG), thromboxane B2(TXB2) and fibrinopeptide A(FPA). In this study, weobserved a significant increase in plasma PF4 and BTG in blood samples withdrawn through catheter afterangiography, compared with those just after catheterization and with those of venous blood samples. Also weattempted to measure directly the thrombi formed in or around angiographic catheter by using arterial sheath in anexperiment with 5 dogs and a considerable decrease in amount of thrombi was observed by flushing catheter andsheath with heparin or contrast media, in comparison with those by flusing with normal saline.