Satellite cells were isolated from skeletal muscles of adult normal and dystrophic mice (C57/6J/dy strain) by sequential digestion of tissue fragments with collagenase, hyaluronidase and trypsin. These cells exhibit in culture similar behaviour to that of embryonic myoblasts, undergoing an initial duplicative period lasting about 2--3 days, followed by a shorter phase (1--2 days) of rapid cell fusion. During the duplicative phase most of the satellite cells appear round-shaped, whereas embryonic myoblasts appear typically spindle-shaped: both cell types actively incorporate [3H]thymidine. During the subsequent days of culture an increasing number of satellite cells becomes spindle-shaped; afterwards the cells contact each other and fuse into multinucleated myotubes. The majority of spindle-shaped satellite cells is unable to incorporate [3H]thymidine, thus behaving as post-mitotic cells. Concomitantly with satellite cell fusion, an increase of about 80-fold of creatine phosphokinase (CPK) specific activity is observed. Satellite cells are able to recognize co-cultured embryonic myoblasts ([3H]thymidine-labelled): hybrid myotubes containing labelled and unlabelled nuclei are formed in these experimental conditions. Satellite cells from dystrophic animals are able to differentiate in culture and do not show appreciable differences as compared to their normal counterparts. In dystrophic myotubes, however, CPK specific activity is almost twice that observed in normal myotubes. Human dystrophic satellite cells from biopsies of adult muscle cultured in similar conditions grow and fuse into multinucleated myotubes showing a behaviour identical to normal controls.