Male mice of the C3Hf strain were exposed to 600 R of acute X-rays and, along with unirradiated control males, mated with 101-strain females. The offspring of the treated males were all conceived more than 7 weeks after irradiation, thereby ensuring that they were derived from germ cells exposed as stem-cell spermatogonia. After weaning, the offspring were caged individually and allowed to live their normal lifespan. Tumors and other major pathological disorders were recorded at a careful post-mortem examination. The lesions encountered were typical of those characteristically seen in aging (101 x C3Hf)F1 mice. The results showed no significant differences in lifespan between experimentals and controls. This held true when allowance was made for littermate correlations and for other factors that might contribute to differences among litters. Likewise, there were no significant differences between experimentals and controls in the frequency, severity, or age distribution of neoplasms and other diseases.