The following human low density lipoproteins were prepared: ß-lipoproteins of densities greater than 1.040 (A, B,C) a ß-lipoprotein of – S 1·063 = 5 (D), a lipoprotein of – S 1·063 = 19 (E), and a lipoprotein of – S 1·063 = 70 (F). Data are presented which show the immunochemical homogeneity of the D lipoprotein rabbit-anti-D lipoprotein system. Cross-reactions between antibody to A and D lipoproteins and the above lipoproteins have been demonstrated by quantitative precipitation, quanitative complement fixation, and single and double diffusion in agar. The antigenic similarities appear to be associated with the protein portions of the molecule. The antisera produced did not differentiate the low density lipoprotein classes.