A retina cell membrane protein that is distributed in a dorsal---(arrow) ventral topographic gradient in retina was purified 3000-fold. We have tested the hypothesis that signal transduction through cell membranes is dependent on phosphatidylcholine synthesis by transmethylation and find that stimuls-secretion coupling and signal transduction through cell membranes at synapses are profoundly inhibited by inhibition of the CDP-choline pathway for phosphatidylcholine synthesis but not the transmethylation pathway. Monoclonal antibodies were obtained which are specific for the inner or outer synaptic layer of the retina, for certain types of neurons in retina, for antigens localized in clusters at synaptic end-plates of striated muscle cells and for different types of striated muscle cells. Two antibodies increase 45Ca++ uptake and acetylcholine secretion by neuroblastoma hybrid cells. Another antibody binds to lipomodulin, a protein inhibitor of phospholipase A2. Cells with a clonally inherited defect in synapse formation were shown to be deficient in nicotinic acetycholine receptor aggregation protein and large dense-core vesicles. The defect was partially repaired by the addition of the receptor aggregation protein to the medium A protein from bovine brain was purified almost to homogeneity which induces neurite extension by chick embryo cerbral cortex neurons at nM concentrations. Neurons were isolated from chick or rat embryo spinal cord which synthesize acetycholine and survive only in the presence of muscle cells or a factor secreted by muscle cells into the medium.