The effect of age on the formation of radiation-induced micronucleated reticulocytes (MN-RETs) and reticulocytes (RETs) was investigated by exposing female C57BL/6J mice to graded doses of gamma rays from a (137)Cs source. Age at time of irradiation was 6, 16, or 32 weeks, and doses ranged from 0.5 to 3 Gy. A flow cytometric technique based on anti-CD71 labeling was used to measure RET and MN-RET frequencies in blood specimens collected 43 h post-irradiation. Mean RET frequencies declined in a dose-dependent manner for each age group. There was only one significant difference among the ages, that is, %RETs were not significantly reduced in the oldest animals at 0.5 Gy, whereas this dose did have a significant effect on the other age groups. MN-RET data were more complex. Age was observed to influence the baseline frequency of MN-RET, with the oldest mice exhibiting a significantly higher mean value. Each group's %MN-RETs values increased up to 1 Gy, but past this dose the frequencies plateaued or decreased. Age was observed to influence micronucleus frequency, with older mice exhibiting higher mean MN-RET values, especially at the high doses where the response was saturated (2-3 Gy). We hypothesize that these dissimilar responses can largely be explained by an age-related down-regulation of apoptosis whereby younger animals eliminate damaged bone marrow erythroid precursors with a greater efficiency compared with aged mice.