The effects of 3-aminobenzamide (3 ABA), an inhibitor of poly(ADP-ribose) synthetase, on UV-induced SOS responses were studied in Escherichia coli. The lethal effect of UV irradiation was enhanced by 3 ABA in an excision repair defective strain (uvrA) and in the parental wild-type strain. In contrast, in a recombination defective mutant (recA), 3 ABA inhibited UV-caused cell mortality. An apparent depression of UV-induced mutation by 3 ABA was observed in uvrA. In addition, induction of the umu⁺ gene expression by UV was depressed by 3 ABA in both uvrA and wild-type strains. Prophage induction from λ-lysogenized cells by UV was also depressed by 3 ABA. Taken together, these results demonstrate that 3 ABA inhibits a diverse set of SOS responses induced by UV irradiation.