Two series of 100 subjects each, males and females, have been studied for the determination of plasma values of apo-A1, the major peptide of HDL, and of apo-B, the major peptide of LDL. To minimize a possible influence of variations in plasma lipid levels, two series of subjects were selected with very similar mean values of plasma cholesterol and triglyceride. Despite this criterion of selection apo-A1 was significantly lower in males (127 +/- 18 mg/l) than in females (137 +/- 15 mg/l) while the reverse was true for apo-B (99 +/- 16 mg/l in males vs. 91 +/- mg/l in females). Both apo-B and apo-A1 showed a tendency to increase with advancing age with the greatest increase for apo-A1 (in both sexes) and for apo-B (only in males) from the 3rd to the 4th decade of age.