The following compounds induced prophage lambda in the Escherichia coli WP2(s)Lambda Microscreen assay: adriamycin, m-AMSA, ellipticine, nalidixic acid, oxolinic acid, paraquat, hydrogen peroxide, and sodium azide. Actinomycin D, novobiocin, teniposide, and potassium superoxide did not induce prophage lambda. An inhibitor of DNA gyrase subunit B (novobiocin) does not induce prophage. In contrast, poisons of DNA gyrase subunit A (nalidixic acid and oxolinic acid) were the most potent inducers of prophage among the agents examined here. The next most potent agents were the mammalian DNA topoisomerase II poisons that are reactive intercalators and generators of active-oxygen species (adriamycin and ellipticine). Agents that produce reactive-oxygen species only (hydrogen peroxide and paraquat) were next in potency. The mammalian DNA topoisomerase II poison m-AMSA was the weakest inducer. The results illustrate the relative effectiveness of agents that induce prophage by various mechanisms. Nonetheless, these agents may induce prophage and SOS response by producing essentially the same type of DNA damage, i.e., DNA strand breaks. The study and a review of the literature suggest that certain agents may induce their genotoxic effects in bacteria by mechanisms that are different than those by which they induce their genotoxic effects in mammalian cells.