Monoclonal antibody 30.2A8 was produced by a hybridoma made by fusing cells from rats that had been immunized with rat-human muscle cell hybrids. The 30.2A8 reacts with a differentiation antigen in human skeletal muscle that is synthesized by myoblasts but not myotubes. The gene controlling synthesis of the antigen recognized by 30.2A8 was found to be encoded by human chromosome 15.