Survival curves of Chinese hamster cells exposed to graded doses of sulfur mustard are sigmoid; they start with a shoul der and become exponential. As in the case of X-irradiation, this suggests that cells must accumulate damage for a lethal drug effect, and, therefore, survivors are initially damaged sublethally. In spite of the fact that the survival dependence on position in the growth cycle differs for sulfur -mustard and X-rays, in both cases surviving cells repair sublethal damage rapidly

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... as they proceed to progress in their growth cycles. (This is clear with asynchronous as well as synchronized cells.) Further, as in the case of X-irradiation, the repair of sublethal sulfur mustard damage can be demonstrated readily at a tempera ture (24°C) at which very little cell progression occurs. Although sulfur mustard may not be typical of cancer chem otherapy drugs, and cultured Chinese hamster cells may not be typical of cells of interest in man, these results demonstrate the possible importance of repair of damage when therapy in volves fractionated drug treatments.