Treatment of casein with formaldehyde changes its tertiary structure and decreases its hypercholesterolemic properties in rabbits. To investigate whether formaldehyde-treated casein exerts this hypocholesterolemic effect in the same manner as soybean protein, rabbits were fed high or low cholesterol diets containing soybean protein, casein, formaldehyde-treated casein or a mixture of casein and formaldehyde-treated casein. Formaldehyde-treated casein was hypocholesterolemic when fed in a low, but not in a high, cholesterol diet. The hypocholesterolemic effect of soybean protein was independent of the amount of cholesterol included in the diet. In contrast to rabbits fed soybean protein, steroid absorption in those fed formaldehyde-treated casein did not differ from that in rabbits fed native casein. Furthermore, the absorption of phosphorus and nitrogen was lower in rabbits fed formaldehyde-treated casein than in those fed native casein, whereas the absorption found in rabbits fed soybean protein resembled that of their casein-fed counterparts. The diets containing soybean protein and formaldehyde-treated casein produced a comparable ratio of lysine to arginine in serum. The results presented in this paper indicate that the hypocholesterolemic action of dietary formaldehyde-treated casein does not resemble that of soybean protein.