Adventitial red blood cells produce intimal platelet accumulation in cerebral arteries of cats following subarachnoid hemorrhage. After dividing 21 cats into three equal groups, we exposed their right middle cerebral arteries transorbitally and adventitially irrigated them with 2 ml washed red blood cells, blood plasma, or saline. To determine arterial intimal platelet accumulation in each cat, we injected [111In]oxine-labeled platelets intravenously immediately before injecting the various experimental solutions. Animals were sacrificed 2 or 4 hours following the injection of labeled platelets. Irrigation with washed red blood cells produced a significantly greater intraluminal accumulation of platelets than irrigation with saline (p less than 0.05). Plasma tended to have less of an effect on platelet accumulation than washed red blood cells, but this difference was not significant. These data suggest that the adventitial blood fraction responsible for intimal platelet accumulation in cerebral arteries following subarachnoid hemorrhage may be derived mainly from the red blood cell fraction. 