X-rays and nitrogen mustard: independent action in Chinese hamster cells.

We have studied the combined effects of X-irradiation and nitrogen mustard treatment on the colony-forming ability of Chinese hamster cells. In contrast to X-irradiation, nitrogen mustard acting by itself yields an exponential dose-effect relationship. Moreover, whether delivered in immediate sequence or after varying intervals, nitrogen mustard treatment does not affect radiation survival and vice versa. These results, plus the lack of any indication of the repair of sublethal nitrogen mustard damage, are consistent with independent but different modes of action at the same site(s), or action at different sites.