We investigated age-related changes in normal human diploid fibroblasts (strain AG 1522) followed throughout their lifespan in vitro. Radiosensitivity and the capacity to repair potentially lethal radiation damage (PLD) remained constant until senescence was reached. However, the effects of incubation with lactate on PLD repair were age dependent and complex. Low millimolar concentrations of lactate increased the capacity to repair PLD, whereas higher concentrations suppressed it in a concentration-dependent manner. With increasing in vitro age, the cells became less sensitive to the modifying effects of lactate; in presenescent cells, the modifying effects of lactate on PLD repair had completely vanished.