The effect of dietary fiber isolated from dehulled, defatted soybean seeds on cholesterol (CHOL) metabolism and atherosclerosis in rabbits was studied alone and in combination with isolated soy protein (ISP). Soy fiber (SF) contains both cellulosic and non-cellulosic dietary fiber. Based on the official AOAC method, soy fibers contains 75% total dietary fiber. Rabbits at 6 months of age were randomly assigned to 1 of 4 dietary treatments. All rabbits received either a casein or ISP-based diet with cellulose or SF as the only dietary fiber source for 36 weeks. Fasting blood samples and feces were collected and analyzed for lipids from individual rabbits. The entire aorta was removed and fixed, and sudanophilic stained lesions were examined visually. Rabbits consuming the SF and/or ISP diets had lower plasma CHOL levels and lower incidence of atherosclerotic lesions relative to the rabbits fed the casein-based cellulose diets. Rabbits consuming the SF and/or ISP diets also had a lower CHOL content in their liver and heart. Rabbits fed ISP-based diets had consistently increased fecal bile acid excretion, whereas rabbits fed diets containing SF had increased fecal and cholesterol concentration. These results suggest a complementary role for SF and ISP in preventing atherosclerosis in rabbits.