The radiosensitivity of the first cell cycle in mouse lymphocytes was compared to that of the later cell cycles by means of analysis of chromatid-type aberrations. A nearly pure population of first-division metaphases (M1) was obtained when lymphocytes were harvested 28 h after stimulation. When the cells were fixed at 52 h of culture time, approximately 90% of the metaphases were composed of M2 and M3 or later cells. After irradiation with X rays 24 or 48 h after stimulation, metaphases were scored from sequential samplings. Generally, no significant differences in aberration frequencies were found between cells irradiated at the two times, indicating that the radiosensitivity of mouse lymphocytes does not vary with their age in culture after stimulation with phytohemagglutinin. Significantly different aberration yields were observed at different sampling times, emphasizing the necessity of sequential samplings when analyzing aberrations in cells irradiated during asynchronous growth.