Alterations in the negative charge of isolated nuclei after X-irradiation were investigated by electrophoresis in three different radiosensitive cell lines. The electrophoretic mobility decreased with time after irradiation and reached a minimum 4 hr after exposure. The relationship between the decrease in mobility measured 4 hr after irradiation and the radiation dose was linear and independent of cell lines utilized. In Burkitt lymphoma cells P3HR-1 (n = 1,$D_{0}=67$ R) the decrease in mobility was irreversible: The surving fraction of cells (colony-forming ability) and the fraction of electrophoretically intact nuclei 4 hr after exposure showed good correlation. In mouse melanoma cells B16-C2W (n = 20,$D_{0}=160$ R), however, the nuclear charge recovered almost completely by 24 hr after irradiation with doses up to 500 R, and good correlation was indicated between the surviving fraction (colony-forming ability) and the fractions of "electrophoretically intact" nuclei isolated 24 hr after exposure for five dose-points.

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