The ability of S3 HeLa cells to divide during continuous irradiation was studied using time-lapse cinemicrography. Cells were initially synchronized by mitotic selection, and the irradiation was started 2 hr later. For dose rates of approximately 35 rad/hr very few cells were able to divide during 60 hr of continuous irradiation, but many accumulated for prolonged periods in a rounded configuration. Parallel studies in which cells were harvested and fixed for microscopic examination showed that most of the rounded cells seen in the time-lapse films must have been cells which entered mitosis, attempting to divide, and not cells rounding and dying from an interphase configuration. After 50 to 60 hr of continuous irradiation the frequency of cells with micronuclei increased sharply. Mitotic accumulation during continuous irradiation was similar in some respects to that seen in the presence of colcemide.

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