The Bowman-Birk inhibitor (BBI) is a soybean-derived serine protease inhibitor with anticarcinogenic activities. BBI, in the form of BBI concentrate (BBIC), is currently being evaluated in clinical trials as a human cancer-preventive agent. In the present study, an enzyme-linked immunosorbent assay was used to measure BBI concentrations in serum samples collected from human subjects and animals treated with BBIC. The results demonstrate that the serum BBI concentration was higher than the baseline level for the patients after treatment with BBIC at 100-800 chymotrypsin-inhibitor units/day for 0.5, 1, 2, 4, and 6 mo. The increase in serum BBI concentration was also observed in dogs treated with BBIC at 100-1,000 mg/kg/day for 52 wk, and the increase was dose dependent. The results also indicate that anti-BBI antibodies were present in animals and the serum levels of anti-BBI antibodies increased significantly in mice treated with BBIC at 100-1,000 mg/kg/day for 15 and 26 wk. The increase in the serum level of anti-BBI antibodies in dogs treated with BBIC was not statistically significant, and no increase in the serum level of anti-BBI antibodies was observed in human subjects after BBIC treatment. These results suggest that orally ingested BBI is absorbed by human subjects and animals and that some animals develop antibodies to BBI in response to treatment with BBIC.