Chromosome damge induced by X-irradiation or bleomycin was measured using the cytokinesis-block micronucleus assay in the peripheral blood lymphocytes of 6 newborn, 8 young and 10 elderly individuals. An increase in the frequecy of spontaneous micronuclei with age was observed. There was no difference in the X-irradiation-induced micronucleus frequency between the 3 groups. There was a significant increase with age in the number of micronuclei induced by bleomycin. Kinetochore-labelling studies revealed that the percentage of kinetochorone-positive induced micronuclei was higher for bleomycin (36.2-43.3%) that for X-irradiation (17.1-19.7%). The age-related increase in frequency of spontaneous or bleomycin-induced micronuclei was due to increases in both kinetochrone-positive and kinetochrone-negative micronuclei. The frequency of kinetochrone-positive or -negative micronuclei induced by X-irradiation was not different between the 3 age groups. There results suggest that bleomycin is more potent in inducing whole-chromsome loss than X-rays and that lymphocytes from aged individuals are more sensitive to bleomycin in terms of both chromosome breakage and whole chromosome loss.