By means of the modified Cohn-method 6 Cohn-IV-1-fractions were prepared in the small pool technique as sterile, pyrogen-free and HDL-rich serum fractions (HDLF), the HDL-concentrations of which were 15-20 g/l. In in-vitro investigations they inhibited the spontaneous aggregation of erythrocytes as well as the ADP-induced aggregation of thrombocytes and activated the molar and fractional initial cholesterol estirification rates as well as the prostacyclin synthesis. From HDLF intravenously applied in people as immediate effects result significant decreases of the cardiac pre- and afterload, an increase of the cardiac ejection fraction as well as a complete dissolution of circulating aggregates of thrombocytes. In 35 patients with hyperlipidaemias after infusion of 100 ml HDLF up to the 5th day increasing and for 14 days remaining increases of concentration of biliary bile acids, total phospholipids and of cholesterol were observed in lithogenic indices remaining in the normal region. The plasmatic lipo- and apolipoproteins reacted with increases, decreases or constancy of concentration which might be based on the different ethryopathogeneses of the hyperlipoproteinaemias.