Currently available ultracentrifugal procedures for separating the two major density subfractions of HDL (HDL2 and HDL3) are time-consuming and expensive, and require relatively large volumes of plasma. In order to overcome these problems, a rapid and simple procedure, requiring only 0.5 ml plasma, has been developed. After precipitation of apoprotein B-containing lipoproteins with heparin and MnCl2, 120 microliters of the supernatant is adjusted to a density of 1.125 g/ml with aqueous NaCl-NaBr, and centrifuged for 3.5 h at 160000 x g in an air-driven bench-top ultracentrifuge (Beckman Airfuge). The HDL2 is then removed in a tube-slicer, constructed to permit six Airfuge tubes to be sliced simultaneously, and the cholesterol content assayed by a cholesterol oxidase procedure. The method provides an alternative to the recently described heparin/MnCl2/dextran sulphate double-precipitation procedure, with the advantage of permitting quantitative recovery of intact HDL2.