The present study was designed to observe the effects of soyprotein and casein with or without cholesterol on serum and liver lipids in male rats. The 6 experimental groups were as fellows ; SF ; soyprotein, cholesterol-free diet. SC ; soyprotein, 0.5% cholesterol added diet. CF ; casein, cholesterol-free diet CC ; casein,0.5% cholesterol added diet. SCF ; protein mixture of soyprotein and casein(1 : 1), cholesterol-free diet SCC ; mixed protein, 0.5% cholesterol added diet. The hypocholesterolemic and hypotriglyceridemic effects of soyprotein were observed at 3 weeks, but these effects disappeared at 6 weeks. The hypocholesterolemic effect of soyprotein was more obvious when the 0.5% cholesterol was supplemented in the diets. The serum free cholesterol level was not affected by the dietary protein source or the dietary cholesterol, therefore, the difference in serum total cholesterol among groups seems due to the difference in cholesterol esters. There was a tendency of a higher percentage of HDL in soyprotein groups compared to casein groups at 1 week, however, this tendency disappeared with time. The liver cholesterol and triglyceride contents were not differ among cholesterol-free diet groups, however, with addition of cholesterol, those of soyprotein groups were significantly lower than casein groups. The higher serum arginine/lysine ratio of soyprotein groups may offer the part of explanation of its hypocholesterolemic effect.