A study was conducted in seven males and eight females to determine the effect of feeding 25 and 35% fat calories on plasma lipids, blood clotting parameters and platelet aggregation. A statistically significant reduction in thrombin- and collagen-induced aggregation was observed at the end of the 25 %-fat-calorie period. At the end of the 35 %-fat-calorie period the thrombin- and collagen-induced aggregation tests rebounded to the predietary level. Platelet counts rose significantly and white blood cell counts decreased by the end of the 25 %-fat-calorie period. Both platelet counts and white blood cell counts returned to predietary levels at the end of the 35 %-fat-calorie period. Plasma cholesterol decreased an average 15% at the end of the 25 %-fat-calorie period and remained at the lowered level throughout the 35 %-fat-calorie period. It is suggested that levels of total and saturated fat in the diet are significant factors affecting the aggregation tendency of blood platelets.