The X-ray survival curve for asynchronous Chinese hamster V-79 cells at 17-20 hr after plating when cells are irradiated as microclones of two to four cells, differs from the survival curve seen at short times after plating, when single cells are irradiated, in having higher D0 (300 rad vs 160 rad) and negligible extrapolation number. As a consequence of the difference in D0 the difference in survival between single cells and clones increases with increasing dose. Transient cyclic changes in survival occur at early times after plating and are probably related to partial synchronization induced by trypsinization. In addition there is a progressive increase in survival which develops with increasing time after plating, as the number of cells in the clones increases. Decrease in radiosensitivity with increasing number of cells irradiated is also observed for synchronous cells when cells at corresponding points in the cell cycle are irradiated. Accumulation and repair of sublethal damage is demonstrable in cells irradiated at short times after plating, but cannot be shown at 20 hr after plating when cells are irradiated as microclones.

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