Monoclonal antibodies are being used to identify skeletal muscle cell surface proteins and to define the approximate distribution and number of each protein species in the sarcolemma of muscle cells in tissue culture and in vivo. The long-term goal is to know the skeletal muscle plasma membrane and basement membrane in quantitative molecular and functional terms and to know the mechanisms of their biogenesis and organization. During the proposed research year, sarcolemma antigens are to be characterized by physicochemical terms and turnover rates measured for the plasma membrane components. The cellular origin of each basement membrane component is to be determined and studies continued on the organization of basement membrane components and their interactions with plasma membrane. Several such components have been discovered to be concentrated at neuromuscular junctions and at sites of acetylcholine receptor clustering on tissue cultured muscle cells. The elaboration of these specially organized regions of muscle surface is being studied.