A monoclonal antibody, 5.1H11, directed against human fetal muscle and myogenic cells in tissue culture, was used for immunofluorescence analysis of frozen sections of muscle biopsies from 24 patients with different diseases of muscle. The staining pattern was highly specific; detectable levels of the 5.1H11 antigen were restricted to regenerating myofibers as assessed by comparison with serial sections stained with hematoxylineosin. There was no appreciable staining of intrafusal, normal adult extra fusal, denervated, degenerating, or necrotic muscle fibers. Thus, the 5.1H11 antibody allows unambiguous identification of regenerating myofibers in biopsy specimens.