Injection of the myotoxin notexin, was found to induce regeneration in muscles that had been subjected to 18 Gy of radiation. This finding was unexpected as irradiation doses of this magnitude are known to block regeneration in dystrophic (mdx) mouse muscle. To investigate this phenomenon further we subjected mdx and normal (C57Bl/10) muscle to irradiation and notexin treatment and analysed them in two ways. First by counting the number of newly regenerated myofibres expressing developmental myosin in cryosections of damaged muscles. Second, by isolating single myofibres from treated muscles and counting the number of muscle precursor cells issuing from these over 2 day and 5 day periods. After irradiation neither normal nor dystrophic muscles regenerate to any significant extent. Moreover, single myofibres cultured from such muscles produce very few muscle precursor cells and these undergo little or no proliferation. However, when irradiated normal and mdx muscles were subsequently treated with notexin, regeneration was observed. In addition, some of the single myofibres produced rapidly proliferative muscle precursor cells when cultured. This occurred more frequently, and the myogenic cells proliferated more extensively, with fibres cultured from normal compared with dystrophic muscles. Even after 25 Gy, notexin induced some regeneration but no proliferative myogenic cells remained associated with the muscle fibres. Thus, skeletal muscles contain a number of functionally distinct populations of myogenic cells. Most are radiation sensitive. However, some survive 18 Gy as proliferative myogenic cells that can be evoked by extreme conditions of muscle damage; this population is markedly diminished in muscles of the mdx mouse. A small third population survives 25 Gy and forms muscle but not proliferative myogenic cells.