Inhibitors of UV induction of the SOS function were screened. A log phase culture of E. coli PQ37 (sulA::lacZ, rfa, uvrA, Phoc) was irradiated with UV and then immediately subjected to culture for 2 h in a liquid LB medium containing each test compound. Expression of the SOS gene (sulA) was assayed by monitoring the levels of beta-galactosidase. In order to examine the inhibitory effects of test compounds on protein synthesis, the levels of the constitutive alkaline phosphatase were assayed in parallel. The total number of compounds tested was 233, including 44 food and feed additives, 23 naturally occurring compounds and derivatives, 21 antibiotics, 61 pesticides, 33 inorganics and 51 other chemicals. As a result, 5-fluorouracil and 5-fluorodeoxyuridine were found to inhibit considerably the UV induction of the SOS gene without any inhibition of protein synthesis. Mutagenesis induced by UV irradiation was depressed by the addition of either compound at non-toxic concentrations.