The effects of excess cystine added to diets with casein, egg protein, soya-bean protein and wheat gluten as protein source on liver and serum lipids of rats were compared. 2. The addition of excess cystine to a soya-bean protein diet produced lipid accumulation in the liver. 3. The addition of excess cystine to casein, egg protein and soya-bean protein diets, but not a wheat gluten diet, increased serum cholesterol.