We have investigated the time of appearance of the earliest differentiating glial cell types of human spinal cord using a panel of antigenic markers to identify them in cultures from 6- to 9-week-old human embryos. Immunolabeling performed at 14 h in vitro with the O4 mAb, an early oligodendrocyte marker, showed the presence of oligodendrocytes during the 7th week of age. At 8 weeks only a few of the O4+ cells expressed galactocerebroside (GalC), a marker of more differentiated oligodendrocytes. All the O4+ and GalC+ cells were vimentin+ and some of the GalC+ cells were A2B5+, GD3+ and SSEA-1+. During the first week in vitro many of the O4+ cells exhibiting a more immature, bi- or tri-polar morphology incorporated [3H]thymidine into their nuclei. Cells expressing the astrocyte-specific marker GFAP could be first observed at 8 weeks; almost all of these GFAP+ cells, which should correspond to radial glia on the basis of the current literature, were vimentin+, A2B5+, GD3+, and SSEA-1+. At 2 days in vitro incorporation of [3H]thymidine could be shown in a small fraction of these cells. The finding that radial glia and oligodendrocytes expressed similar antigenic features and the additional observation that a small, but consistent fraction of the cells were simultaneously labeled by O4 and anti-GFAP antibodies support the hypothesis that, in the human spinal cord, radial glial cells can give rise to both oligodendrocytes and astrocytes; in this respect, radial glial cells may be similar to the A2B5+, GD3+, vimentin+ bipotential glial progenitors previously identified in cultures from developing rat CNS, which also express A2B5, GD3, and vimentin.