Low density human plasma lipoproteins Sf 17+, Sf 13, and Sf 6, high density lipoproteins 2 and 3, and a lipoprotein-free plasma protein fraction were isolated from human plasma by ultracentrifugal methods. It was found that human plasma lipoproteins are immunochemically distinct from the non-lipoprotein containing plasma protein fraction. Lipoprotein fractions of a given hydrated density, isolated from different individuals, were found to be immunochemically indistinguishable by qualitative absorption tests. Qualitative antigenic differences were shown to exist between low density lipoproteins and high density lipoproteins. Quantitative precipitin reactions showed that low density lipoproteins Sf 6 and Sf 13 were immunochemically very similar. However, they differed with respect to the amount of antigen nitrogen required for maximum precipitation. Agar diffusion analyses were performed; the results suggest heterogeneity of lipoproteins by this criterion.