Plateau phase Chinese hamster cells (HA-1), growing in monolayers, form a convenient system for studying the repair of potentially lethal damage. Cells are irradiated under conditions of density inhibition, and the interval between irradiation and explant (to assay survival) is varied. Repair is evidenced by higher survival of those cells maintained in plateau. This increase in survival can be expressed as a slope-modifying factor. The magnitude of the slope-modifying factor is determined by the cells' post-irradiation environment. Cells overlaid with Hanks' BSS in the interval between irradiation and explant showed a maximum slope-modifying factor (∼1.3), cells in serum free or "conditioned" medium a somewhat lower value, while minimum repair was seen if the cells were in fresh medium supplemented with fetal bovine serum. Hypoxia does not seem to affect the magnitude of repair of potential lethal damage if the comparison of survival increase is made at similar survival (rather than dose) levels. Cells in G1 repair potential lethal damage actively and account for most, if not all, of the increase in survival. The rate of repair has a half-time of 1-2 hours, both in aerobic and in hypoxic cells. Repair is essentially complete after 6 hours. Exponentially growing cells show little or no evidence of repair under similar conditions and over a similar time interval.

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