Spores and vegetative cells of Bacillus subtilis strains with various defects in DNA-repair capacities (hcr-, ssp-, hcr-ssp-) were irradiated with UV radiation or X-rays. Induced mutation frequency was determined from the observed frequency of prototrophic reversion of a suppressible auxotrophic mutation. At equal physical dose, after either UV- or X-irradiation, spores were more resistant to mutations as well as to killing than were vegetative cells. However, quantitative comparison revealed that, at equally lethal doses, spores and vegetative cells were almost equally mutable by X-rays whereas spores were considerably less mutable by UV than were vegetative cells. Thus, as judged from their mutagenic efficiency relative to the lethality, X-ray-induced damage in the spore DNA and the vegetative DNA were equally mutagenic, while UV-induced DNA photoproducts in the spore were less mutagenic than those in vegetative cells. Post-treatment of UV-irradiated cells with caffeine decreased the survival and the induced mutation frequency for either spores or vegetative cells for all the strains. In X-irradiated spores, however, a similar suppressing effect of caffeine was observed only for mutability of a strain lacking DNA polymerase I activity.