IN PART III of our series of communications on idiopathic hyperlipemia and primary hypercholesteremic xanthomatosis* the observation was reported that in both diseases a single intravenous injection of 50 mg. of heparin caused a decrease in most of the elevated serum lipid values within 15 minutes. In primary hypercholesteremic xanthomatosis, in which the values for cholecterol and phospholipids are elevated and the plasma is clear, we found in 10 patients a slight but consistent decrease in the amount of cholesterol: from an average value of 444 mg. to one of 417 mg. per 100 ml. of blood serum. In idiopathic hyperlipemia, in which also the values for neutral fat are elevated and the plasma is milky, we found in seven patients a consistent decrease in the degree of turbidity and in the amounts of cholesterol and phospholipids, and in five patients a decrease also in