It has been shown previously that the radiation sensitivity of LexA strains of Escherichia coli K-12 can be suppressed by thermosensitive mutations (designated tsl) that are closely linked to the lexA locus. These are thought to be intragenic suppressors that reduce the activity of the diffusible product that gives rise to the LexA- phenotype (Mount et al., 1973). When a recA mutation is crossed into a suppressed tsl- strain, the extreme radiation sensitivity usually conferred by a recA mutation is considerably reduced without any detectable change in genetic recombination deficiency. Suppression of UV sensitivity depends upon the activity of the uvrA+ product. We propose that at least part of the radiation sensitivity of a recA- strain is due to a DNA repair defect that is different from inability to perform genetic exchanges and depends upon the presence of the lexA+ product. We hypothesize that the lexA+ product is a repressor of the synthesis of repair enzymes. In recA+ cells with DNA lesions, repressor is inactivated leading to enzyme induction but this does not occur in recA- cells. tsl mutations inactivate repressor leading to constitute enzyme synthesis and bypassing the need for recA+ product to inactivate the lexA+ product.