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 In this study we observed a significant increase in plasma PF4 and BTG in blood samples withdrawn through catheter after angiography compared with those just after catheterization and with those of venous blood samples.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