Primary cultures prepared from human muscle biopsies were examined for the presence and distribution of acetylcholine receptors, as measured by the binding of 125iodine-labelled alpha-bungarotoxin. The toxin bound to the human muscle cultures with a similar time dependence and specificity as found in muscle cultures from other species. The amount of toxin bound was lower than that obtained for neonatal mouse muscle under similar conditions. The distribution of receptors was similar in cultures derived from the muscles of patients with a variety of neuromuscular disorders. The toxin was located along the myotubes in a fairly even distribution; however, variations in labelling along a single myotube were observed, as well as variations between different myotubes in the same culture. Occasionally, the toxin also bound to other cells, which may have been mononucleated. Cultures prepared from patients with Duchenne muscular dystrophy produced multi-layered cell clusters, instead of the usual monolayer of cells. Within these clusters, only the myotubes bound alpha-bungarotoxin.