The initial effect of sulfur mustard on a dividing population of L cells was inhibition of DNA synthesis. The progression of G2 and G1 cells was not immediately affected, so that the result was an accumulation of cells that synthesized DNA slowly. After about 24 hours the cells contained, on the average, the premitotic amount of DNA, but cell division did not occur. This level of DNA was maintained for at least another 60 hours. Cells that had been released from an FUDR block and treated with mustard at or near the time of release exhibited a purely exponential survival curve. If the release was delayed relative to the mustard treatment, survival was enhanced. The survival curve obtained under these conditions was parallel to the former curve but exhibited a shoulder. When a fractionated-dose technique with mustard was used no evidence for repair of sublethal damage was found; rather, the two doses of mustard, separated in time, were more toxic than a single equivalent dose.

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