1. Plasma high-density lipoprotein (HDL) was separated by heparin Sepharose affinity chromatography into a non-bound, apolipoprotein E-poor, and a bound, apolipoprotein E-rich, fraction through the binding effect of Mn2+ in the column buffer. 2. The application of a series of elution buffers in which the concentration of Mn2+ was progressively replaced by Mg2+ resulted in the separation of the bound HDL into five subfractions. 3. Each subfraction migrated a different distance on gradient-gel electrophoresis. Three of the subfractions had R(F), (relative migration compared with BSA) values within the range of HDL(2b). One subfraction contained largely HDL(2a), with some material in the regions of HDL(2b) and HDL(3a), and one subfraction spanned the R(F) regions of HDL(2a), HDL(3a) and HDL(3b). 4. The number of molecules, per HDL particle, of cholesteryl ester, non-esterified cholesterol and phospholipid increased with particle size, whereas triacylglycerol passed through a maximum and the number of amino acid residues remained approximately the same. 5. Apolipoprotein (apo) A-I was the major apoprotein in all five subfractions, but the latter differed appreciably in their contents of apo A-II and apo E. 6. The major fatty acid component of each subfraction was linoleic acid, with moderate amounts of C(16:0) and C(18:1) fatty acids and a smaller content of C(18:0), C(20:4,n-6) and C(22:6,n-3) with no significant difference in composition between the subfractions. 7. This paper provides the first description of a method for the isolation of three subfractions of HDL(2b) together with other subfractions in quantities that are sufficient for further analytical or metabolic studies.