An ultracentrifugal technique for separating and analyzing serum lipoproteins was evaluated in comparison with analyses by electrophoresis using agarose-gel and polyacrylamide-gel. In general, the percent of pre beta- and beta-lipoproteins in electrophoresis was estimated higher than the percent of VLDL and LDL in ultracentrifugal method, while the percentage of alpha-lipoprotein in the former was estimated lower than that of HDL in the latter. In cases with abnormal lipoproteinemias, various discrepancies arose between the methods. For examples, pre beta- and beta-lipoproteins were estimated too high by the analyses with electrophoresis. The cholesterol content in HDL decreases in hypertriglyceridemia accompanied by an increase in triglyceride content. Therefore, when HDL cholesterol is determined by a polyanion method to assess the net HDL concentration in such cases, it is estimated to be low. Such errors are not only found in the determination of HDL cholesterol, but also in apoproteins in liver cirrhosis, because the composition of HDL apoprotein is markedly altered. Since the heterogeneity of lipoproteins separated by ultracentrifugation is characteristic in hereditary disorders of lipoproteins such as LCAT deficiency, the centrifugal technique is essential for lipoprotein analysis in such disorders. The disadvantages in ultracentrifugation are cross-contaminations among fractions, and removals of lipids and apoproteins from lipoprotein particles. Apo A-I and E proteins, and phospholipids were removed from the particles more rapidly than other components. From the results of repeated ultracentrifugation of HDL, 3% of apo A-I was estimated to be lost from the HDL during the centrifuge procedure.