Exposure of plateau phase cells to 5× 10-5 M 2,4-dinitrophenol (DNP) in complete medium during X irradiation delivered at 80-230 rad/min greatly enhanced their survival. The D0 for H4 (rat hepatoma) cells was increased from about 140 to 250 rad, and for αRST (a mouse-rat hybrid strain) from 240 to 355 rad. Survival was further increased if the cells were allowed to repair potentially lethal damage after irradiation by delaying subculture. A similar enhancement of survival by DNP was found in the three aneuploid cell lines studied, but no effect could be demonstrated in two non-transformed, contact-inhibited mouse fibroblast lines (3T3 and 10T-1/2). Exposure to DNP under identical conditions had no influence on survival in any of the cell lines following high dose-rate γ-irradiation (95-220 rad/sec). Split-dose recovery was very rapid in plateau phase cells irradiated at the high dose-rates; a threefold enhancement in survival occurred when 3-5 min elapsed between doses. No significant recovery occurred ...