Abstract Levels of S100, a protein characteristic of nerve supportive tissue, undergo two distinct transitions during the development of avian embryonic spinal ganglia. Radioimmunoassay shows that both transitions also occur in cultured spinal ganglia. However, homogeneous cultures of supportive cell precursors from 5-day embryonic spinal ganglia fail to undergo the second developmental transition marked by large increases of S100. Immunohistochemistry, utilizing peroxidase-anti-peroxidase procedures, reveals that homogeneous supportive cell cultures stain lightly and uniformly for S100 whereas whole ganglion cultures contain some intensely stained cells, which are only associated with neuron cell bodies or fibers. Neurons, therefore, may be required to induce high S100 levels in differentiating supportive cells.