The various stages of divergent neuroepithelial differentiation were studied in the solid transplants of a transplantable mouse testicular teratoma (OTT-6050) maintained in both ascitic and solid forms. They included: a) areas of undifferentiated medullary epithelium corresponding to the rare human medulloepithelioma; b) areas of neuroblastic differentiation corresponding to neuroblastoma, with more mature neuronal differentiation corresponding to ganglioneuroma or, when mixed with glial elements, to ganglioglioma; and c) more mature neuroglial areas resembling astrocytoma, oligodendroglioma or ependymoma, as well as more primitive areas corresponding to ependymoblastoma. In tissue culture using collagen-coated coverslips, astrocytic differentiation was found in the outgrowth zone after 15 days, confirmed by immunofluorescence with antibodies to an astroglia-specific protein. In organ culture systems, glial components, including ependymal structures, were preserved in tumor explants, and astrocytic differentiation, as expressed by glial fiber formation, was increased after 4 to 6 weeks in vitro. No neuronal differentiation was demonstrable, however. The neuroepithelial component of this experimental teratoma may provide a model for the study of neoplastic neuroepithelial differentiation.