Mice containing log phase S-180 (Sarcoma 180) ascites tumor cells were given whole-body X-irradiation (50, 100, 200, 400 R) to produce mitotic inhibition (block in G2). The effect of caffeine (1.9, 19, 194, 1942 μg) was studied by administration to the mice before X-irradiation (5, 10, 20, 40 min). The dose-related period of mitotic arrest was reduced when caffeine was administered 5 or 10 min prior to X-irradiation. This effect of caffeine was no longer evident when the period before X-irradiation was extended to 20 min. There was no such caffeine effect when administered after X-irradiation. The duration of mitotic inhibition was diminished at all concentrations of caffeine employed.

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