Basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) are mitogens for bipotential oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells. We investigated the mitogenic effect of these growth factors on quiescent mature oligodendrocytes (OL) expressing myelin basic protein (MBP) in OL cultures that were treated for 3 days with cytosine arabinoside (ARA-C) in order to kill O-2A precursors which divide in chemically defined medium. After treatment with ARA-C proliferation decreased and O-2A precursors identified with A2B5 monoclonal antibody were nearly undetectable. After exposure of mature OL to bFGF, cell proliferation increased markedly within 24 hr. PDGF had a much weaker effect. Cultures treated with ARA-C for 3 days and then with bFGF for the next 24 hr and incubated with BrdU for the last 2 hr before the end of the experiment were immunolabeled with anti-MBP or A2B5 and anti-bromodeoxyuridine (BrdU) antibodies. Eighty-seven percent of the cells were MBP+, 10% were both MBP+ and BrdU+, and none was A2B5+ BrdU+, showing that at least a part of the population of mature MBP+ OL retains the ability to reenter the cell cycle in vitro. Since mature OL did not proliferate in response to bFGF in the cultures not treated with ARA-C, i.e., in the presence of O-2A progenitors, we assumed that these precursors were responsible for the lack of mitogenic effect of bFGF on MBP+ OL in such conditions. Conditioned medium from O-2A precursors almost halved the bFGF-induced OL proliferation after treatment with ARA-C, suggesting that O-2A progenitors control the proliferation of a subpopulation of mature OL (possibly young mature OL) via the secretion of active molecule(s).