The lethal and mutagenic effects of 7 alkylating agents: N-nitroso-N-methylurea (NMU), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), nitrogen mustard (HN2), mitomycin C (MC), bifunctional acridine mustard (AM)--and of cyanate (KNCO) on heat inducible lambda cI857 prophage were studied. After treatment of lysogenic cells with mutagens, prophage was heat-induced either immediately or after 90 min incubation in nutrient broth and c mutants forming clear plaques at 32 degrees C were scored. NMU (0.02 M) when immediately induced with heat, induces c mutants very efficiently (maximal yield 10%) not only in the wild-type cells but also in repair-deficient mutants recA13, lexA102, uvrA6 umuC36, recF143, xthA9, polA1, uvrD3 and uvrD502. These data show that NMU-induced mutations are fixed as replication errors due to mispairing modified bases. After delayed heat induction, the prophage survival enhances and the frequency of c mutations declines considerably in host cells of all repair genotypes tested. Carbamoylation is not involved in the mutagenic action of NMU, because KNCO (0.02 M) has a very slight lethal effect and does not induce mutations. MNNG (100 micrograms/ml) and EMS (0.1 M) also induce mutations by replicative mechanism, because maximal yield of c mutations does not depend on RecA+ and is about 15 and 2%, respectively. MMS is a mutagen of the repair type, since its mutagenic action is suppressed by recA mutation of the host. NH2 only inactivates prophage, but does not induce mutations. MC (50 micrograms/ml) and AM (150 micrograms/ml) induce mutations rather inefficiently (the maximal yield 0.1 and 0.3%, respectively) both in recA+ and recA- hosts. The mutagenic action of these agents is probably due to intercalation.