To understand how the differentiation of stem cells to oligodendroglial progenitors is regulated, we established cultures of neural stem cells from neonatal rat striatum in the presence of epidermal growth factor (EGF) as free-floating neurospheres that were then exposed to an increasing amount of B104 cell-conditioned medium (B104CM). The resultant cells proliferated in response to B104CM but no longer to EGF. In vitro analysis and transplantation studies indicated that these cells were committed to the oligodendroglial lineage, and they were thus referred to as oligospheres. Further characterization of their expression of early markers, cell cycle, migration, and self-renewal suggests that they were pre-O2A progenitors. RT-PCR analysis indicated that the oligosphere cells expressed mRNAs of platelet-derived growth factor alpha receptor in addition to fibroblast growth factor receptor but not EGF receptor; the latter two receptor mRNAs were expressed by neurosphere cells. Thus, the progression of stem cells to oligodendroglial progenitors is likely induced by factors in B104CM.