Two mutations have been located at the recA locus and phenotypically characterized along with a third one, previously called rec-34. The three mutants behaved similarly to lexA mutants. They were sensitive to ultraviolet (UV) light and X rays, and lambdaFec- phages were able to plate on them. The three mutations were called lexB because they could be distinguished from recA mutations by the last property. lexB mutants were less sensitive to UV and X irradiations than were recA mutants and were, to various degrees, recombination proficient. UV light failed to induce prophage lambda in all three lexB lysogens. In contrast, thymine starvation induced lexB31 and lexB34 lysogens. In lexB34 mutants, but not in lexB30 and lexB31 mutants, UV reactivation occurred at a low level. In Escherichia coli K-12, the recA gene has basic functions in the repair of deoxyribonucleic acid lesions, deoxyribonucleic acid recombination, and prophage induction. The three lexB mutations alter unequally and independently the three functions. This suggests that the recA and lexB mutations affect the same gene.