The radiation responses of two heterogeneous murine tumor lines in vitro was studied at the time of X-irradiation. The proliferating (P-cells) 66 and 67 cells displayed equal radiosensitives, however, the quiescent (Q) cells were considerably more radiosensitive than the P-cells, with the 66 Q being more radioresponsive than the 67 Q. A detailed analysis of the metabolic status vs cell-cycle age (i. e., G,1, vs S-phase) indicates that the cell-cycle age was the predominat influencing factor for radiation-induced responses. The data also showed that in the plateau phase Q-cell cultures pH and cell density were not influencing factors.