Embryonic (E10) mouse cerebellum and spinal cord cells were cultured and infected with simian virus 40 (SV40). In all infected dishes, rapid proliferations (a dividing time less than 18 h) of non-neuronal cells were detected. Three populations of those cells, SVa, SVb and SVc, could be passaged more than 10 times. They had flat morphologies like young astrocytes and, indeed, showed immunoreactivities to glial fibrillary acidic protein (GFAP), a marker for the astrocyte. However, they had an ability to form colonies in a soft agarose medium. The large-T antigen could be detected in nuclei of the three cell populations by a fluorescent antibody test and immunoprecipitation. No transformation of neuronal cells was detected, nevertheless there were neuronal cells still possessing dividing capacities in the culture preparation.