Certain protease inhibitors are effective at preventing or suppressing carcinogen-induced transformation in vitro and carcinogenesis in animal model systems. One protease inhibitor, the soybean-derived Bowman-Birk inhibitor (BBI) is particularly effective in suppressing carcinogenesis. BBI is a protein of a molecular weight of 8000 with a well-characterized ability to inhibit trypsin and chymotrypsin. BBI has been extensively studied, both as purified BBI and as an extract of soybeans enriched in BBI called BBI concentrate (BBIC). Purified BBI and BBIC have comparable suppressive effects on the carcinogenic process in a variety of in vivo and in vitro systems. BBI appears to be a universal cancer preventive agent. Purified BBI and BBIC suppress carcinogenesis as follows: in 3 different species (mice, rats, and hamsters); in several organ systems and tissue types [eg, colon, liver, lung, esophagus, cheek pouch (oral epithelium), and cells of hematopoietic origin]; and in cells of epithelial and connective tissue origin when given to animals by several different routes of administration, including the diet, leading to different types of cancer (eg, squamous cell carcinomas, adenocarcinomas, and angiosarcomas), and induced by various chemical and physical carcinogens. About half of an oral dose of BBI is taken up into the bloodstream and distributed throughout the body, with excretion via the urine. Pharmacokinetic studies of BBI have been performed in animals with radioactively labeled BBI, whereas antibodies that react with reduced BBI are being used in pharmacokinetic studies in humans. The calculated serum half-life is 10 h in both rats and hamsters. BBIC achieved Investigational New Drug status from the FDA in April 1992 (IND no. 34671; sponsor, Ann R Kennedy), and studies to evaluate BBIC as an anticarcinogenic agent in human populations began. Both BBI and BBIC prevent and suppress malignant transformation in vitro and carcinogenesis in vivo without toxicity.