The concentration of total cholesterol and triglycerides in the 3 major lipoprotein classes of human serum was measured in 136 men, randomly selected from an industrial population, by a quantitative method of lipoprotein electrophoresis on agarose gel and on fractions separated by preparative ultracentrifugation. Correlation coefficients for the 2 estimates were 0.98 for triglycerides in very low-density lipoproteins, 0.93 for total cholesterol in low-density lipoproteins and 0.75 for total cholesterol in high-density lipoproteins. Data obtained from the analyses of the ultracentrifugal fractions were used to develop regression equations that predict the concentrations of total cholesterol and triglycerides in the lipoprotein classes from their concentrations in whole serum. These equations take into account the inverse curvilinear relationship between total cholesterol in high-density lipoproteins and serum triglyceride concentration. When applied to a separate sample of 530 men, predicted values for triglycerides in very low-density lipoproteins and total cholesterol in low-density lipoproteins correlated as well with ultracentrifugal values as did the electrophoretic estimates. For total cholesterol in high-density lipoproteins, the electrophoretic method was superior. Similar regression equations were developed from ultracentrifugal lipoprotein analyses in 158 women from the same industrial population. Although the concentration of total cholesterol in the low-density lipoproteins estimated by both electrophoresis and the regression equations agreed closely in most cases with the ultracentrifugal values, errors exceeded 10% with sufficient frequency to limit the value of the estimates for this purpose. In both men and women, the ratio of total cholesterol to triglycerides in high-density lipoproteins was a hyperbolic function of serum triglyceride concentration, suggesting that cholesteryl esters in the core of this lipoprotein are progressively replaced by triglycerides as the concentration of triglycerides in very low-density lipoproteins increases. This altered composition of nonpolar lipids accounts, at least in part, for cholesterol reduction in high-density lipoproteins in hyperlipemic individuals.