We have previously shown that repeated or continuous intimal injury caused by an indwelling aortic catheter causes a variety of lesions in rabbits maintained on a diet unsupplemented by lipid. These include fatty streaks, lipid-free fibrous plaques and lipid-rich raised thromboatherosclerotic plaques. Whether lipid-rich raised lesions are a result of injury or co-existing thrombosis or both is not clear. The present experiment was designed to answer this question. Anti-platelet serum (APS) to washed sonicated rabbit platelets was raised in sheep. PE 60 polyethylene catheters were placed in the aortas of 35 rabbits by way of a femoral artery. The animals were randomly divided into 2 groups. The experimental group (17 rabbits) received an intravascular injection of 1.0 ml of APS followed 8 hours later by a subcutaneous injection of 0.5 ml. Thereafter, 0.5 ml APS was given subcutaneously each day for 13 additional days. The control group (18 rabbits) received no APS. Platelet counts were done prior to surgery, at 5 minutes following surgery, at 4 days, 8 days and just prior to killing. Extent of lesions was estimated by photographing the opened aortas, projecting the photographs on cardboard, cutting out the areas occupied by the different lesions and weighing the cardboard. The mean weight of raised lesions in the control group was 6 to 7 times greater than in the experimental groups. Statistical analysis of this difference based on Welsh‘s “t” test for unequal variances was highly significant (P<0.001). Platelet counts in the experimental group varied from 0 to 20,000 at 14 days. In animals with platelet counts ≤ 1,000 mm3 raised lesions were completely prevented. In a second experiment the effect of APS was compared with normal sheep serum (NSS). A similarly significant inhibition of raised lesions occurred in the APS group. The extent of lesions in the NSS control was similar to that in the No-APS group of the first experiment. These findings indicate that thrombosis is more important than injury in the development of lipid-rich raised lesions.