The repair of potentially lethal damage following treatment by gamma radiation was investigated in human fibroblasts held in a non-cycling state by maintenance in a medium containing 0.5 per cent foetal calf serum. Variation in their capacity to repair PLD was noted between three normal cell strains. A failure to repair PLD in ataxia-telangiectasia cells (AT5BI) was confirmed. In three cell strains which were intermediate between normals and A-T cells in their sensitivity, XP3BR, 46BR and GM1142, a limited capacity for the repair of PLD was observed. Two other cell strains, 47BR and 67BR, which showed little if any hypersensitivity could be clearly distinguished from normals after a 24 hour period for the repair of PLD. Thus the technique might permit better discrimination between cell strains. One other cell strain, H15617, could be distinguished from normals by proving hypersensitive under all conditions. Here, however, the repair of PLD appeared to be normal.