The uncoupler of aerobic oxidative phosphorylation, 2,4-dinitrophenol (DNP), has been previously shown to decrease the radiation sensitivity of L cells. In the present investigation we performed paired irradiation experiments with DNP-treated cells. Although DNP treatment virtually stops macromolecular synthesis, the paired-dose survival curve showed a definite "repair" segment. The absolute survival reached by the DNP-treated cells was greater than that in parallel irradiated control cells kept in complete growth medium.