The Bowman-Birk inhibitor (BBI) is a soybean-derived anticarcinogenic protease inhibitor with anti-inflammatory activity. To assess the possibility of utilizing BBI for alleviating the side effects associated with lung cancer radiation and chemotherapy, we have determined the effects of BBI and a soybean concentrate enriched in BBI (known as BBIC) on radiation- and cis-platinum-induced cytotoxicity in A549 human lung cancer cells. The results demonstrated that neither BBI nor BBIC protected A549 cells from radiation- and cis-platinum-induced cytotoxicity. In fact, BBI and BBIC potentiated the cell-killing effects induced by cis-platinum alone, and BBIC treatment led to significantly enhanced cell killing by cis-platinum in combination with radiation treatment in the lung carcinoma cells. BBI conferred a significant protective effect onto mouse fibroblasts (10T1/2 cells) treated with cis-platinum in combination with 6 Gy of X-ray irradiation. These results suggest that BBI and BBIC, when given to lung cancer patients, are unlikely to interfere with cancer treatment utilizing radiation and cis-platinum.