Previous biochemical studies on DNA repair competence and aging have been limited to techniques, such as alkaline elution or nucleoid sedimentation, involving mass cell populations. These techniques provide no information about the distribution of DNA damage and repair among individual cells and are unlikely to detect age-dependent changes affecting a minor fraction of the cell population. We have recently described a microgel electrophoretic assay (Singh et al., 1988) that measures, at the level of the individual cell, single-strand DNA breaks and alkali-sensitive sites. Here, we employ this method to analyze DNA damage and repair in lymphocytes isolated from the peripheral blood of 31 subjects (23 males and 8 females aged 25-91 years) and exposed in vitro to 200 rads of X-irradiation. While basal (pre-irradiation) levels of damage were independent of the age of the donor, an age-dependent increase in DNA damage was observed immediately following irradiation. For all subjects, the mean level of DNA damage was restored to pre-irradiation control levels within 2 h of incubation at 37 degrees C. However, a distribution analysis of DNA damage among cells within each sample indicated the presence of a few highly damaged cells (4-16%) in the 2-h sample, the occurrence of which was significantly more common among aged individuals. These data indicate an age-related decline in DNA repair competence among a small subpopulation of lymphocytes.