Chemical studies of broad band lipoproteins isolated from fasting sera show wide ranges of lipid composition for different classes of lipoproteins, but establish relative constancy of lipid composition for lipoproteinsof the same class obtained from different individuals in both health and certain disease states. Moreover, chemical studies of narrow band Sf 4-8 lipoproteins show no significant differences in lipid content when compared with thecomposition of broad band St 0-20 lipoproteins. Following intravenous heparin injection, lipid compositions of the various lipoprotein classes show some changes in unesteri-fied cholesterol and unesterified fatty acid content. _ Also, nearlyone-fifth of the total serum lipids are shifted, after heparin injection, into associations with proteins present in the total ultracentrifugal “residue.” Studies involving the in vitro incubation of Sf 20-400 lipoproteins with “active fractions” obtained from heparinized plasma show similar, if not identical, lipoprotein transformations as those found to occur in vivo.