Spinal cord neurons form stable synapses on muscle cells in culture, whereas retinal neurons, a n inappropriate presynaptic partner for muscle cells, form synapses that are transient. We have hypothesized that a trophic influence of neurons on muscle is involved in the stabilization of synapses. Because other neural tissues that form stable synapses on muscle cells contain factors that aggregate acetylcholine receptors (AChR) into clusters on the surface of muscle cells, it may be that these aggregation factors are necessary for stabilization of neuron-muscle synapses. Therefore, we determined the AChR-aggregating activity of retinal neurons. The results showed that cocultures of retinal neurons and muscle cells and retinal-conditioned medium do not show increases in the number of AChR on muscle cells. Conversely, spinal cord-muscle cocultures and spinal cord-conditioned medium produce increases in the number of AChR clusters. These data, along with previous studies demonstrating that retinal neurons are unable to affect the electrical membrane properties of cultured muscle cells, whereas spinal cord neurons do elicit such changes, add support to the above hypothesis of a trophic influence of neurons in synapse stabilization.