Noncholesterol marine sterols, abundant in molluscan shellfish, could inhibit cholesterol absorption. Eight normolipidemic males were fed for 3 weeks each three natural food diets in which a mixture of oysters and clams, crab, or chicken was served as the primary source of animal protein. The diets were equalized for caloric distribution, cholesterol, and n-3 fatty acids, leaving the noncholesterol marine sterols in the oyster/clam diet (444 mg/2,000 kcal) as the potential lipid-modifying variable. Cholesterol absorption was measured by plasma isotope ratio after doses of oral 14C- and intravenously infused 3H-labeled cholesterol. Cholesterol absorption was lower (42 +/- 4%) during the oyster/clam diet than during the chicken (54 +/- 3%, P less than 0.01) or crab (55 +/- 3%, P less than 0.01) diet periods. There was no difference between the chicken and crab diet periods. Total plasma cholesterol and triglycerides, very low density lipoprotein cholesterol and triglycerides, and low density and high density lipoprotein (HDL) cholesterol were not significantly different between any of the diets. The ratio of the HDL2-/HDL3-cholesterol was higher following the oyster/clam diet (0.46 +/- 0.09) than the chicken diet (0.32 +/- 0.06, P less than 0.05). Plasma and red cell membrane n-3 fatty acids were not significantly different among the three diets, but red cell membrane n-3 fatty acids increased as a function of time regardless of dietary sequence.