Previous studies have shown that polyunsaturated ruminant fats in the diets of human subjects cause an increase in cholesterol and bile acid excretion during the first 3 weeks of such diets. The present studies were designed to compare the effects of polyunsaturated (P) and conventional (S) ruminant fats at two levels of dietary cholesterol intake: a higher (HC) and lower (LC). Four study periods, each of about 3 weeks' duration, were conducted in 5 healthy subjects providing these dietary combinations: HCS, HCP, LCS, LCP. Neutral sterols and bile acids were measured in the feces, and sterol balances were calculated. Plasma cholesterol levels were significantly lower with P than with S diets at both HC and LC intakes. Changes attributable to differences in fatty acids and to differences in cholesterol intake appeared to exert independent effects. The major changes occurred in lipoproteins with density 1.019-1.045. Cholesterol absorption expressed as a percentage of the dietary intake was not significantly different with the four diets. Neutral sterol excretion of probable endogenous origin and bile acid excretion were significantly higher during the HCP than during the HCS periods, but the difference between LCP and LCS periods was less marked. Net sterol excretion was therefore significantly greater with HCP and LCP than with HCS and LCS diets, the differences being greater at HC than at LC intakes. Comparisons of diets with similar fatty acid but differing cholesterol intakes showed lower net sterol excretion with HCS than with LCS diets (presumably due to suppression by HC by cholesterol synthesis), but this difference was not seen between HCP and LCP diets. This finding, together with greater sterol excretion with HCP than with HCS diets, showed that enhanced sterol excretion with polyunsaturated fat was potentiated with higher cholesterol intake. This enhanced excretion was generally greater during the first than during the second 3-week period of polyunsaturated fat.