A high level of glucose-6-phosphate dehydrogenase (G6PD) activity was observed in myoblasts and myotubes from normal human and mouse cell cultures. However, only a residual amount of activity was observed in myoblasts and myotubes obtained from G6PD-deficient patients (G6PD Mediterranean). Hybrids were formed by the fusion of normal (from human and mouse) and G6PD-deficient myoblasts (from the patients). These hybrids contained a high level of G6PD activity. Hoechst staining permitted to confirm that the enzymatic activity was not restrained to a domain near the competent nuclei. These results suggest that myoblast transplantation could be used to restore normal enzymatic activity in metabolic myopathies.