In a recently-characterized ethidium-bromide-sensitive E. coli strain, DNA appears to be much more accessible to DNA-binding agents. This strain therefore appears to be of interest for studying the mutagenic properties of chemicals. For this purpose, a series of ethidium-sensitive E. coli strains (Ebs) with normal and defective DNA-repair capacity was constructed and made lysogenic for lambda (sfiA::lacZ). These strains were used to study the cytotoxicity and SOS-inducing ability of ethidium and its two photoactivable analogs 8-azido- and 3,8-diazido-ethidium. When non-covalent DNA complexes are formed, these dyes elicit only a bacteriostatic effect in the Ebs strains, which is almost independent of the strain's DNA-repair capacity. The SOS system is not induced. When covalent DNA adducts are formed after photoactivation of ethidium azido analogs, the effects are quite different. The formation of about 5 DNA monoadducts per cell induces a lethal hit in the Ebs uvrB recA strain and measurable SOS induction in the Ebs uvrB (lambda (sfiA::lacZ) strain. The formation of more than 1000 DNA adducts in the Ebs strain with normal DNA-repair capacity does not induce any measurable cytotoxic effect.