Serum apolipoprotein A-I (apo A-I) and lipoprotein cholesterol and triglycerides were measured in 289 persons randomly selected from a Northern California industrial population in 1974-1976. Apo A-I and high density lipoprotein (HDL) cholesterol were strongly correlates with one another and both were inversely correlated with very low density lipoprotein (VLDL) triglycerides. The decrease in HDL-cholesterol with increasing VLDL-triglycerides was relatively much larger than the concomitant decrease in apo A-I. The relative decrease in the sum of cholesterol and triglycerides in the HDL fraction was similar to that for apo A-I, suggesting that the decreasing HDL-cholesterol:apo A-I ratio with increasing VLDL-triglycerides is due in large part to reciprocal transfer of cholesteryl esters for triglycerides between HDL and VLDL. Mean apo A-I level was 16 mg/dl higher in women not taking exogenous sex steroids than in men, 31 mg/dl higher in women taking estrogens without progestins and 10 mg/dl higher in contraceptive drug users than in other women, and 8 mg/dl higher in black than in white men. The first two of these differences were statistically significant. Apo A-I level was unrelated to age, but increased with ethanol consumption and decreased with adiposity. An inverse relationship between Apo A-I and cigarette smoking was found among women.