The present paper shows a series of experiments carried out to elucidate the possible mechanism by which soybean protein isolate (SPI) produces a lower level of plasma cholesterol than casein in rats fed a cholesterol-free diet. When the plasma cholesterol level was in a steady state characteristic of casein and SPI, SPI in the diet was substituted for casein and vice versa. Within 3 days after substitution of dietary protein, the plasma cholesterol level in each group reached a steady state level similar to that in its previous counterpart. The inherent responses of plasma cholesterol to casein and SPI were not changed by the resection of the jejunum or the ileum and the administration of cholestyramine or beta-sitosterol. The rates of the sterol synthesis in vivo of the liver and the small intestine were significantly higher in SPI-fed rats than in casein-fed rats. The hypocholesterolemic effect of SPI disappeared when Met was supplemented at a level equivalent to casein. The effects of casein and SPI were reproduced by their equivalent amino acid mixtures. The ratio of the postprandial increment of Met concentration to that of Gly concentration in the portal plasma was significantly higher when casein and its amino acid mixture were fed than when SPI and its amino acid mixture were fed. The casein-induced increase in the level of plasma cholesterol was attributed to an increase in high density lipoprotein (HDL)-cholesterol. HDL-cholesterol concentration showed the positive correlation with the lecithin:cholesterol acyltransferase activity.(ABSTRACT TRUNCATED AT 250 WORDS)