Killing of log-phase V79 Chinese hamster lung cells by ionizing radiation is enhanced when cells are treated immediately after irradiation with anisotonic phosphate-buffered saline or with medium containing 90% D2O. These treatments are not toxic to unirradiated cells. In medium containing D2O, cells are unable to repair sublethal damage; repair occurs after a shift to normal medium. As with anisotonic buffer [Utsumi, H., and Elkind, M.M.], enhanced killing by medium containing D2O does not prevent cells from repairing sublethal damage. Further aspects of the enhanced expression of potentially lethal damage by medium containing D2O and anisotonic buffer are similar. Both are largely ineffective after treatments involving nonionizing radiation, and both treatments retard the repair of DNA breaks. These qualities and the partial overlapping of enhanced killing due to medium containing D2O and due to hypertonic saline imply that the latter treatments affect the repair or reparability of the same sensitive targets.