In this project we are studying dynamic aspects of the behavior of sarcolemma during muscle differentiation in vitro, and are attempting to correlate changes in surface dynamic properties to the expression of muscle-specific behavior. This research includes studies of the lateral mobility of acetylcholine receptors (AchR), the role played by such mobility in the topographic rearrangement that AchR undergo during muscle differentiation, and the regulation of AchR topography by membrane fluidity, cytoskeletal components, and innervation. Additionally, the relationship between myoblast fusion and membrane lipid dynamics is to be examined. Noteworthy among our methods of procedure are two experimental strategies which permit the quantitative study of AchR and lipid dynamics on single intact living muscle cells. One strategy is based on fluorescence polarization principles and the other is based on resonance energy transfer. Both approaches rely on nanosecond time-resolved fluorescence measurements performed through a microscope, an approach that for brevity we term FRM (Fluorescence Relaxation Microscopy). For these studies AchR are labelled with fluorescent conjugates of alpha-Bungarotoxin.