We have analysed in detail the properties of oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells derived from the spinal cords of adult rats to gain further insights into the mechanisms that control the generation of oligodendrocytes in the healthy and demyelinated adult central nervous systems (CNS). When O-2A progenitor cells from adult spinal cord are exposed in vitro to the AA homodimeric form of platelet-derived growth factor (PDGF-AA), they express a unipolar morphology, an O4-positive, vimentin-negative antigenic phenotype, divide at slow rates, and appear to generate oligodendrocytes by asymmetric division and differentiation. Furthermore, exposure of these cells to PDGF-AA is sufficient to stimulate their proliferation at clonal density. When adult spinal cord O-2A progenitor cells are exposed simultaneously to PDGF-AA and basic fibroblast growth factor (PDGF/bFGF), they are almost completely inhibited from differentiating into oligodendrocytes, divide more rapidly than cells treated with PDGF-AA, and express a bipolar morphology and an O4-negative, vimentin-positive antigenic phenotype. These findings indicate that adult spinal cord O-2A progenitor cells resemble in many aspects their well-characterised adult optic nerve counterparts. In addition, evidence is presented to indicate that neurotrophin-3 (NT-3) is not mitogenic for adult spinal cord O-2A progenitor cells and that it does not enhance their proliferative response to PDGF-AA or PDGF/bFGF. Since relatively large numbers of O-2A progenitor cells can be obtained from adult spinal cord, it should facilitate the further characterisation of these cells.