We have used the P19 line of mouse embryonal carcinoma (EC) cells to initiate studies on the putative role of the int-1 proto-oncogene during neuronal differentiation. P19 cells are induced to differentiate into neurons, astrocytes and fibroblast-like cells following exposure to retinoic acid (RA). Treatment of the same P19 cells with dimethyl sulfoxide (DMSO) leads to differentiation into mesodermal derivatives, including skeletal and cardiac muscle. Northern blot analysis showed that int-1 RNA was not present in the EC cells but appeared 48 h after RA exposure and could be detected for at least the next 8 days. No int-1 RNA was detected in P19 cells induced to differentiate with DMSO. Nuclear run-on transcription assays showed that int-1 expression in RA-treated P19 cells was induced at the transcriptional level. Immunofluorescent staining with an antibody directed against an int-1 peptide identified immunoreactive material in cytoplasmic granules of fibroblast-like cells in RA-treated P19 cultures. Thus the P19 cell line is a suitable experimental system to study int-1 gene expression and function during neuroectodermal development.