The lithogenic diet routinely used for production of gallstones in hamsters contains 20% casein. It has previously been shown that replacement of casein by soy protein significantly decreases gallstone formation. In this study hamsters were fed a lithogenic diet containing casein (C), soy isolate (S), C/S 3:1, C/S 1:1, and C/S 1:3. The percentages of hamsters with gallstones on these five diets were: 44, 12, 38, 23, and 15. Biliary cholesterol levels and lithogenic index both decreased significantly with increasing levels of soy protein. Dilution of casein with soy protein progressively decreases lithogenicity.