Virgin mice of 6 weeks of age, mature mice of 4+ months, and old breeders of 14+ months of age were given an acute whole body exposure to X-rays of 300 r and complete blood analyses at 48 hours, 2, 4, 6, 8, 10 and 12 weeks to determine any differences in recovery related to age. RBC recovery was poor in the old mice, which were anemic; WBC counts remained depressed in all 3 groups, even at 12 weeks. Hemoglobin, platelets, neutrophils were all permanently depressed more among the older mice. Lymphopenia for the old mice settled at 49%. Multinucleate lymphocytes were found in all groups before X-irradiation and exposure did not statistically alter their frequency. An anomalous lymphocyte with satellite or cyto-plasmic inclusion, never found in normal mice, was found in X-irradiated mice and persisted in some for 8 weeks. These may be diagnostic for radiation exposure.