WHEN monolayer cultures of mammalian cells are allowed to reach the stationary or plateau phase of growth, overall DNA synthesis decreases markedly. This is chiefly a result of the appearance of many non-proliferating cells which remain in the presynthetic (G₁) stage of the cell cycle, while the cell concentration remains approximately constant because the lowered rate of cell division is balanced by the sloughing of dead cells into the nutrient medium^1,2. Such cultures are interesting to radiobiologists because they represent an in vitro cell renewal system which has several of the characteristics of human malignant tumours. When Chinese hamster cells were irradiated in the plateau phase of growth, the slope of the survival curve was similar to that obtained with exponentially growing cultures, but the cells did not accumulate and repair sub-lethal damage³. I have investigated the effects of radiation on stationary cultures of a line of cells derived from normal human liver (Chang)⁴. After irradiation in the plateau phase of growth, not only do these human cells recover from sub-lethal radiation damage, but potentially lethal damage is repaired if the cells are allowed to remain in the stationary phase for some time after irradiation.