Elder, Robert L. (Johns Hopkins University, Baltimore, Md.), and Roland F. Beers, Jr. Nonphotoreactivating repair of ultraviolet light-damaged Micrococcus lysodeikticus cells. J. Bacteriol. 89:1225-1230. 1965.-Micrococcus lysodeikticus cells are highly resistant to ultraviolet radiation. The shape of the dose-survival curve is consistent with the presence of a repair mechanism. The presence of nontoxic doses of dinitrophenol (0.1 mm) or iodoacetate (1 mm) before or after radiation decreases the survival number and the initial lag in the dose-kill curve. The extent of decrease in survival after postradiation treatment with these compounds is dependent upon both time and temperature. Organisms that survive radiation show a prolonged lag phase before multiplying. No photoreactivation of the radiated cells can be demonstrated. Ultraviolet-radiated phage show a higher inactivation when plated on host cells of M. lysodeikticus treated with dinitrophenol at concentrations which do not affect the infectivity of nonradiated phage. The resistance of these cells to ultraviolet radiation is the result of a very efficient dark-cell repair mechanism that can be blocked by iodoacetate or dinitrophenol.