The clonal development of oligodendrocytes was studied by culturing individual oligodendrocyte--type-2 astrocyte (O-2A) progenitor cells on monolayers of type-1 astrocytes, which stimulate O-2A progenitor cells to divide. Oligodendrocytes developed by a proliferative lineage in which clonal progeny differentiated together after a number of cell divisions. Most O-2A progenitor cells had similar cell cycle times (1-2 days), but their proliferative capacity varied greatly: some divided only once while others divided up to eight times before differentiating. sister cells behaved similarly when recultured separately on astrocyte monolayers. These findings are consistent with the cell-division-counting hypothesis previously proposed to explain the timing of oligodendrocyte differentiation. They also unambiguously establish the phenotype of O-2A progenitor cells in vitro and demonstrate that these cells respond directly to growth factors produced by type-1 astrocyte monolayers.