Spontaneous ovarian teratomas develop in a large proportion of female LT strain mice. These tumors display a large neuroectodermal component with morphologic differentiation ranging from primitive neuroepithelium (medulloepithelial and ependymoblastic rosettes) to mature neurons, and provide a useful system for the study of various asynchronous stages of neuroepithelial differentiation. The aim of this study was to assess the expression of various cytoskeletal proteins in conjunction with other differentiation-related antigens in these tumors. We found that the medulloepithelial rosettes reacted with only two anti-beta-tubulin monoclonal antibodies. One of these (TU27) recognizes an epitope common to all of the mammalian beta-tubulin isotypes. The other monoclonal antibody (TUJ1) recognizes an epitope unique to class III beta-tubulin isotypes (neuronal-associated). Whereas immunoreactivity in the ependymoblastic rosettes was limited to TU27, differentiating polar neuroblasts reacted with both TU27 and TUJ1 and expressed neuron-specific enolase, synaptophysin, and the 68 kilodalton subunit of neurofilament protein. In well-differentiated foci, mature neurons were positive for all three neurofilament protein subunits (68, 168 and 200 kilodaltons), microtubule-associated-protein 2, synaptophysin, and neuron-specific-enolase, and reacted with both TU27 and TUJ1. By contrast, glial elements expressed glial fibrillary acidic and S-100 proteins, Leu-7 and TU27 but not TUJ1. Myelin basic protein and myelin-associated glycoprotein reactivity was found in the neuropile of these mature areas. The neuroepithelial components were negative for retinal S-antigen and cytokeratin. The expression of the class III beta-tubulin isotype by medulloepithelial rosettes suggests that this isotype may be one of the earliest markers to signal neuronal commitment in primitive neuroepithelium.