A chromatographic fraction obtained from a thymus homogenate of young calf, was administered by intraperitoneal means to a distinct group of rats previously fed on a diet with high content of dietary fat for 50 days. The serum values of cholesterol and triglycerides and of total lipids in spleen, kidneys and aorta which were obtained from these animals were compared with those of rats fed on the same diet and not injected with the thymus fraction. The considerable decrease of the seric values was not followed by conparable results of the lipidic content in the organs.