Thiourea (TU) is a thyroid carcinogen which has previously been shown to cause genotoxicity in various test systems in vitro and in vivo. The mechanism underlying these effects has not yet been elucidated. The present study addressed the question of whether the formation of oxidized products of TU might be involved in genotoxicity. Chemical oxidation of [14C]TU with hydrogen peroxide in the presence of calf thymus DNA resulted in the formation of [14C]formamidine sulfinate ([14C]FASA), [14C]cyanomide, and [14C]urea and in covalent binding of radioactivity to the DNA. Incubation of V79 Chinese hamster cells with 10-20 mM TU for 18 hr but not for 3 hr, increased the frequency of micronuclei to a slight extent. In cells depleted of glutathione, which can prevent the oxidation of TU, micronucleus induction by TU was more pronounced and detectable both after 3 and 18 hr of incubation. Exposure of the cells to 1.25 to 10 mM FASA for 3-5 hr induced micronuclei, DNA repair synthesis, and gene mutations in the cells. Flavin-containing monooxygenase (FMO], an enzyme known to catalyze the S-oxygenation of TU in liver, could not be detected in the postmitochondrial supernatant (S-9) of the V79 cells. There is evidence, however, that TU can easily autoxidize to S-oxygenated products. Both FASA and TU caused a slight induction of DNA repair synthesis in cultured rat hepatocytes, but FASA was active at lower concentrations than TU. Cyanamide did not elicit repair. The finding that FASA, a product of both the nonenzymatic and the enzymatic S-oxygenation of TU, is genotoxic in cultured mammalian cells provides for the first time a hypothesis to explain the genotoxicity of TU.