Studies in recent years have suggested that measurement of high density lipoprotein (HDL) subclasses may provide significant information beyond that provided by measurement of total HDL. However, conventional methodology for separation of HDL subclasses involves various types of ultracentrifugation that are time-consuming, costly, and not suitable for many clinical or epidemiological studies. We have developed a simple precipitation method for the separation of HDL subclasses in human plasma. After precipitation of apoB-containing lipoproteins with heparin-Mn2+, HDL2 is precipitated by addition of dextran sulfate (mol wt 15,000). HDL2 cholesterol is calculated as the difference between total HDL cholesterol (heparin-Mn2+ supernatant) and HDL3 cholesterol (dextran sulfate supernatant). HDL2 determined by this method correlated well with results obtained by preparative ultracentrifugation (n = 295, r = 0.91) and analytical ultracentrifugation (n = 17, r = 0.92). In the original method final concentration of dextran sulfate was 0.09 g/dl; however further studies indicated that 0.13 g/dl is a more suitable concentration. The chemical compositions of HDL2 and HDL3 isolated by the precipitation method were very similar to those of HDL2 and HDL3 isolated by preparative ultracentrifugation. The concentration of HDL2 cholesterol was 40% higher in normal women than in normal men. In men with coronary heart disease, total HDL was decreased by 28%, HDL2 was decreased by 44%, while HDL3 was 19% lower. A similar pattern of change was found in women with coronary heart disease. In other conditions where total HDL either increased or decreased, the change in HDL2 was always proportionately greater than the change in total HDL. HDL3 showed relatively less change, and in some instances its concentration was unchanged. Thus HDL2 is the more variable component and may be a more meaningful index of altered HDL metabolism.