During embryonic development, neural crest cells differentiate into a wide variety of cell types including Schwann cells of the peripheral nervous system. In order to establish when neural crest cells first start to express a Schwann cell phenotype immunocytochemical techniques were used to examine rat premigratory neural crest cell cultures for the presence of Schwann cell markers. Cultures were fixed for immunocytochemistry after culture periods ranging from 1 to 24 days. Neural crest cells were identified by their morphology and any neural tube cells remaining in the cultures were identified by their epithelial morphology and immunocytochemically. As early as 1 to 2 days in culture, approximately one third of the neural crest cells stained with m217c, a monoclonal antibody that appears to recognize the same antigen as rat neural antigen-1 (RAN-1). A similar proportion of cells were immunoreactive in cultures stained with 192-IgG, a monoclonal antibody that recognizes the rat nerve growth factor receptor. The number of immunoreactive cells increased with time in culture. After 16 days in culture, nests of cells, many of which had a bipolar morphology, were present in the area previously occupied by neural crest cells. The cells in the nests were often associated with neurons and were immunoreactive for m217c, 192-IgG and antibody to S-100 protein and laminin, indicating that the cells were Schwann cells. At all culture periods examined, neural crest cells did not express glial fibrillary acidic protein. These results demonstrate that cultured premigratory neural crest cells express early Schwann cell markers and that some of these cells differentiate into Schwann cells. These observations suggest that some neural crest cells in vivo may be committed to forming Schwann cells and will do so provided that they then proceed to encounter the correct environmental cues during embryonic development.