The survival of colony-forming cells in pig epidermis after irradiation was measured using different electron field sizes. The sensitivity of the colony-forming cells was characterized by D(o) = 2.5-3.0 Gy. There was an effect of field size, described approximately by: Dose (to give five colonies/cm2) (Gy) = (31 +/- 2) x Area-(0.048 +/- 0.015). The effect of field size was less than found previously using tolerance to skin reactions (area exponent = 0.16). This work indicates for the first time that the effects of different large field sizes in skin can be detected at the level of colony-forming cell survival.