Mouse testes were studied histologically 35 or 42 days after irradiation. When doses greater than about 800 rads of γ-rays are given, regeneration of spermatogenic epithelium occurs in discrete foci. It is assumed that a single surviving spermatogenic stem cell can regenerate a spermatogenic focus, and that the absence of spermatogenesis in a tubule signifies the absence of a surviving stem cell in that region of the tubule. The fraction of tubules containing spermatogenic colonies was used as an assay of stem-cell survival. The mean D₀ value of the single-dose survival curve was 180 rads. Survivors of a first dose of 600 rads repair sublethal damage during the first few hours after irradiation. The amount of recovery measured in split-dose studies using intervals of 24 hr to 14 days is less than that measured 4 hr after exposure. This is interpreted as evidence for the existence of phases in the division cycle in which the stem cells are sensitive to radiation and into which a proportion of the first-dose survivors progress. Although surviving stem cells progress through the division cycle, and eventually form a focus of spermatogenesis, regeneration of stem cell numbers was not found in experiments continuing for 14 days after irradiation. The relevance of these characteristics to the development of fractionated-dose radiotherapy is discussed.