Fibroblastic and myogenic cells that retain some of their differential properties can be grown in culture. The former retain density dependent inhibition of growth, while the latter, among other things, sort out and fuse into multinuclear myotubes, develop acetylcholine receptors, become chemically and electrically excitable, and accept innervation from appropriate neurons. The fatty acyl, polar headgroup, and cholesterol composition of the membranes of these cells can be substantially altered. Using several physical techniques, we will characterize the lipid phase from cells with normal and altered membranes. The objective is to determine the structure of the lipid at the growth temperature and to correlate altered membrane functions with altered structure. The interpretations of the physical membrane studies will rely heavily on results from analogous model systems. We will also begin to provide a molecular basis for the role of lipids in myoblast fusion into myotubes.