Serum apolipoprotein B (apo B) levels were found to be significantly (p < 0.001) higher in the 27 patients with combined hyperlipidemia (144 m./dl +/- 27.6) than in the 17 normal weight normolipidemic control subjects (92 mg/dl +/- 20.6; X +/- SD). When compared to apolipoprotein A1 (apo A1) levels obtained in controls (168.5 mg/dl +/- 28.4), hyperlipidemic subjects displayed a moderate yet significant (p < 0.02) decrease of this apolipoprotein (140 mg/dl +/- 24.2). Serum apo B levels were significantly (p < 0.001) correlated with serum cholesterol concentrations and also, to a lesser degree (p < 0.01), with serum cholinesterase activity. A highly significant correlation (p < 0.001) between apo A1 and HDL cholesterol levels was also noted. The decrease ofHDL cholesterol occurring in hyperlipidemic men (-30%) was however more accentuated than the decrease of apo A1 (-18%) suggesting an enhanced transfer of cholesterol esters from HDL to VLDL and LDL. It is considered that the determination of apolipoproteins may be useful not only for the detection of risk factors for atherosclerosis, but also for a better insight concerning the mechanisms involved in the development of an atherogenic dyslipidemia.