In the CBA mouse the radiosensitivity of the undifferentiated spermatogonia that are committed to differentiate was determined by counting their more developed descendants 10 days after graded doses of X rays. Decreasing D0 values were found when these differentiating spermatogonia were derived from undifferentiated spermatogonia that were located in all likelihood in chains of increasing length. In stages IX and X of the epithelial cycle the radiosensitivity of these undifferentiated spermatogonia was characterized by a D0 of 2.2 Gy. This D0 value most likely belongs to the Asingle spermatogonia that form repopulating colonies which give rise to differentiating spermatogonia within the same epithelial cycle. In stages XII/I, where a D0 of 1.0 Gy was found, the dose-response curve is likely dominated by the Apaired spermatogonia present in these stages. In stages III to VII, the Aaligned spermatogonia transforming into A1 spermatogonia determine the radiosensitivity. During this period the D0 decreased from 0.7 to 0.4 Gy. Differentiating A1 to A3 and B spermatogonia had rather similar radiosensitivities of 0.4 to 0.5 Gy.