The sensitivity of epidermal colony-forming units (CFU) in the mouse tail has been compared with the sensitivity of target units responsible for healing the epidermis (EHU). The value of D0 for epidermal CFU was 3.45 +/- 0.36 Gy, much higher than most earlier reports for CFU in dorsal epidermis. The reason for the high D0 is unknown, but it is considered unlikely to be due to marked hypoxia. The value of D0 for EHU was 2.78 +/- 0.51 Gy, which was deduced from the steepness of the dose-incidence curve for the healing of tails. A comparison of the two values of D0 indicates that the inactivation rate of CFU can account for the steepness of the dose-response curve for survival of the tissue. The dose which allowed 50% of tails to heal well corresponded to 3-4 colonies per cm2 of epidermis, to a median peak skin reaction of about 2 (moist desquamation) on an arbitrary scoring scale, and to a slightly lower median skin reaction of 1.7 when the reaction scores were averaged over the period 3 to 6 weeks after irradiation.