To minimize oxidative modification, a low temperature, sequential flotation method was developed to isolate plasma lipoproteins in 18 h using a benchtop ultracentrifuge. The protein distributions were characterized using agarose and SDS-polyacrylamide gel electrophoresis, and an SDS-Lowry protein assay. The lipid distributions were assessed using a gas chromatography-mass spectrometric assay for cholesterol and an enzymatic assay for triglycerides. To validate the rapid flotation method, lipoproteins were also isolated from the same plasma samples using a modified Havel et al. flotation method (J. Clin. Invest. 34: 1345-1353, 1955). The same lipoproteins and apolipoproteins were present in fractions of comparable density, and the summed recoveries of protein, cholesterol, and triglyceride were also identical for the Havel et al. and rapid flotation procedures. Likewise, the amount of cholesterol and triglyceride in corresponding very low, intermediate, and low density lipoprotein (VLDL/IDL and LDL) fractions was the same for the two flotation procedures. The triglyceride and cholesterol levels in high density lipoprotein (HDL) isolated by rapid flotations, however, were 9-12% higher than in the HDL as isolated by Havel et al. Because a 9-12% increase in the HDL fraction reflects only 1-4% of the total triglyceride and cholesterol in plasma, we conclude that, while maintained at 4 degrees C, lipoproteins were quantitatively isolated from human plasma in 1 day.