We used expression of the ganglioside A2B5 to isolate putative myelin progenitor cells from adult human central nervous system parenchyma and compared their phenotypic (expression of myelin lineage molecules) and functional (survival, proliferation) properties with mature oligodendrocytes (OLGs) derived from the same adult material and with A2B5 ؉ cells isolated from human fetal brain. A2B5 ؉ cells represented 3 to 5% of the total cell suspension derived from adult specimens. Results of protein (immunostaining) and RNA (polymerase chain reaction) analyses indicated that the adult A2B5 ؉ cells were more committed to the OLG lineage than their fetal counterparts while continuing to retain properties of progenitor cells compared to the postmitotic mature OLGs. Although the adult A2B5 ؉ cells retained the capacity to divide, albeit at a reduced rate compared to fetal A2B5 ؉ cells, they showed reduced survival and process outgrowth compared not only to fetal cells but also to mature OLGs. Our results confirm the presence of progenitor cells committed to the OLG lineage in the adult human central nervous system but raise the issues regarding the intrinsic capacity of these cells to contribute to the process of remyelination that may be necessary during demyelinating diseases. Recovery from relapses in multiple sclerosis (MS) is attributed at least in part to the extent of remyelination observed in early MS lesions. 16 and PLP ϩ pre-OLGs. 17 These cells do not however appear to be overrepresented nor are they consistently proliferating in the lesions. In late chronic MS lesions, where remyelination is less widespread than in early lesions, the number of both progenitors and mature OLGs is decreased compared to early active lesions.Roy and colleagues, 18 reported that progenitor cells (ganglioside A2B5-positive) comprised 3 to 5% of the total cell population derived from human adult temporal lobe resections and could undergo a limited number of cell divisions in vitro. OLGs could be derived only from the A2B5 ϩ population, although this population could also give rise to neurons and astrocytes. 20 -22 The purpose of our current study was to compare phenotypic (myelin lineage gene expression) and functional (in vitro survival, cell cycling, and process formation) properties of the adult A2B5 ϩ cells with those of mature OLGs isolated from the same tissue samples. We further compared the properties of adult A2B5 ϩ cells with their fetal counterparts providing insight for the observation that, when compared to fetal A2B5 ϩ cells, transplanted adult A2B5 ϩ cells produce fewer surviving cells but among those a higher proportion contribute to myelination of the dismyelinated mouse mutants, shiverer mice. 19 -21 Our results demonstrate that the adult A2B5 ϩ cells retain properties of progenitor cells when compared to postmitotic mature human OLGs and are more committed to the OLG lineage than their fetal counterparts. Adult A2B5 ϩ cells,