Six or 12 months after a single or multifractionated gamma ray irradiation of the skin from the hind legs of mice, the response of the skin to the test dose in single or multi-fractionated (4-16) exposures was studied, using an early skin reaction as an endpoint. The pre-irradiated skin was relatively radioresistant compared to the previously untreated control skin in either dorsal or ventral sides of the legs. The variation in the skin sensitivity depending on the site of irradiation is also discussed for the pretreated and the untreated skin.