Previous studies conducted within the framework of the Lipid Research Clinics Program showed a strong inverse correlation between high-density lipoprotein cholesterol (HDL-C) level and coronary heart disease (CHD) risk in American male populations, whereas in Russian populations such a correlation was less pronounced. It was assumed that HDL was less protective of CHD in Russian than in American males. This study compared the functional activity and lipid composition of HDL, isolated from the blood plasma of men with low, normal, and high HDL-C levels from Moscow (Russia) and Seattle (United States) populations. Results obtained showed that American HDL, irrespective of the plasma HDL-C level, had higher activity in stimulating both [3 H] cholesterol and cholesterol mass efflux from cholesterol-loaded fibroblasts and in suppressing cellular cholesterol esterification when compared with Russian HDL,. American HDL, remained more active than Russian HDL, even when apolipoprotein E-containing particles were removed from HDL, by heparin-Sepharose affinity chromatography. Russian and American 125 I-HDL, had similar binding to high-affinity cell-surface sites, but Russian HDL, had a higher nonspecific binding component compared with American HDL,. This study demonstrates for the first time potential functional differences between HDL particles isolated from Russian and American populations. The lower activity of Russian HDL3 in promoting cellular cholesterol efflux may partly explain the higher CHD risk in the Russian population compared with the American one.