In the present study the effect of feeding the soybean-derived Bowman-Birk protease inhibitor (BBI) on dimethylhydrazine (DHM)-induced gastrointestinal tract and liver carcinogenesis in mice was examined. In this investigation we found the addition of 0.5 or 0.1% semipurified BBI or 0.1% purified BBI to the diet of DMH-treated mice resulted in a statistically significant suppression of angiosarcomas and nodular hyperplasia of the liver and adenomatous tumors of the gastrointestinal tract. Autoclaved BBI or BBI which had its trypsin inhibitory domain specifically inactivated was found to be ineffective in suppressing the induction of these liver and gastrointestinal tract lesions. The results of this study also indicate that BBI, included as 0.5% of the diet or less, has the ability to suppress carcinogenesis with no observed adverse effects on the health of the mice.