In this study, the distribution of the soybean-derived Bowman Birk inhibitor (BBI) in mice was examined. Mice received [125I]BBI by oral gavage and three hours later, the mice were sacrificed, the organs of interest were carefully removed and the distribution of the inhibitor was determined. The bulk of labeled BBI was present in the luminal contents of the small and large bowel, urine and feces. Significant amounts of label were also observed in the serum, esophagus, stomach and intestine, kidney, liver and lung. Analysis of tissue homogenates by gel filtration chromatography revealed that the radioactivity eluted from the column at the same position as the BBI standard indicating that the iodinated BBI was still intact. The chromatographically purified BBI was able to inhibit chymotrypsin indicating that functional protease inhibitory activity was present. These results indicate that the BBI becomes widely distributed in mice 3 h after oral administration and that intact protease inhibitor is present in internal organs.