To assess the cytogenesis of the central nervous system we studied the spinal cord and the cerebrum in 11 human embryos and fetuses of gestation age 7-25 weeks immunohistochemically using anti-vimentin, anti-neurofilament protein (NFP), anti-neuron-specific enolase (NSE), anti-glial fibrillary acidic protein (GFAP), anti-S-100 protein, anti-Leu 7 and anti-myelin basic protein (MBP) antibodies. Vimentin was demonstrated in ventricular cells at 7 weeks and older. NFP-68-kDa and -160-kDa components were observed in neuroblastic cells of the neural tube at 7 weeks. NFP (68 and 160 kDa) was mainly located in the marginal zone of the spinal cord and the cerebrum at 8-9 weeks. NSE was not found in the neural tube at 7 weeks, although NSE was demonstrable at 9 weeks both in the spinal cord and in the cerebrum. GFAP-positive cells started to appear at 9 weeks in the spinal cord and at 15 weeks in the cerebrum, respectively. S-100 immunoreactivity was almost coincident with GFAP. S-100, however, was observed in more numerous glioblastic cells. Leu 7 was detected at 7 weeks and located in the neuropil of the central nervous tissue. MBP was not demonstrable in this study. Our study indicates that neuronal differentiation occurs much earlier than glial differentiation in the human brain and that neuronal and glial cell classes do not coexist in the ventricular zone of the early human fetal brain.