Oltipraz [4-methyl-5-(2-pyrazinyl)-1,2-dithiole-3-thionel, originally developed as an antischistosomal agent, protects against chemical carcinogenesis in lung, trachea, forestomach, small intestine, colon, breast, skin, liver and urinary bladder in rodents. Oltipraz induces electrophile detoxication enzymes, resulting in diminished carcinogen-DNA adduct formation and reduced cytotoxicity, an important component of anticarcinogenic actions. Phase I trials of this drug have been recently conducted in the United States and indicate that the maximum tolerated dose is about 125 mg/day over a six-month period. Grade 1/11 dose-limiting toxicities included photosensitivity/heat intolerance, gastrointestinal, and neurologic toxicities. Ongoing studies are monitoring relationships between dose scheduling, drug plasma concentrations and pharmacodynamic action. Subsequent trials with this agent might most appropriately target individuals at high risk for occupational or environmental exposures to genotoxic carcinogens. Towards this end, a randomized, placebo-controlled Phase I1 study is planned for people at high risk for exposure to aflatoxins and development of hepatocellular carcinoma. Modulation of biomarkers reflecting the biologically effective dose of aflatoxin will serve as study endpoints.