Mutants of bacteriophage T4 which exhibit increased sensitivity to ultraviolet radia-tion were isolated and their abilities to induce an enzyme system to excise pyrimidine dimers were examined. Among 16 mutants isolated, 12 mutants were found to be defective in inducing T4 endonuclease V, which catalyzes the formation of dimer-specific breaks in ultraviolet-irradiated DNA. A leaky v mutant, which exhibits intermediate ultraviolet sensitivity, was also isolated; the mutant induces a low level of endonuclease V and excises only a small amount of dimers in vitro. Three other mutants, as well as x and y mutants, were able to induce both T4 endonuclease V and dimer-excision enzyme (5'→3' exonuclease).