A monoclonal antibody (McAb), 16.3A5, has been characterized that reacts with a cell surface antigen of human muscle cells and a variety of nonmuscle cells. The gene controlling synthesis of 16.3A5 antigen has been assigned to human chromosome 11 by assessing McAb reactivity on a panel of mouse-human cell hybrids. The 16.3A5 has a novel specificity distinct from other chromosome 11-encoded antigens such as W6/34, F10.44.2, TRA1.10, and 4D12 antigens.