The effect of a MLD of total body irradiation on the "embryonic" thymus of the opossum was analyzed. The destructive effect depended on cell types in the thymus when irradiated. Small lymphocytes immediately after formation were more sensitive to irradiation than older small lymphocytes. Hassall's corpuscles and epithelial cells were most resistant to the destructive effect of irradiation. Mitoses were most suppressed at 16 hours after irradiation. The suppressive effect was greatest when the thymus was filled with small lymphocytes at the time of irradiation. "Embryos" irradiated when the mitotic index was highest had an elevated mitotic index. Irradiation of epithelial thymus delayed first appearance of lymphocytes about 2 days. The sequence of appearance of lymphocytes in the "embryonic" thymus after irradiation recapitulated normal embryogenesis. Regeneration of lymphocytes in older "embryos" was modified because of the persistence of small lymphocytes after irradiation. These cells eventually disappeared and the thymus subsequently regenerated as though starting from an epithelial sheet, so also simulating normal embryonic development. The closer irradiation is given to the time of normal appearance of Hassall's corpuscles, the greater the delay in their subsequent appearance. Regeneration from irradiation in embryo recapitulates normal embryology as the latter recapitulates phylogeny.