CNS remyelination and functional recovery often occur after experimental demyelination in adult rodents. This has been attributed to the ability of mature oligodendrocytes and/or their precursor cells to divide and regenerate in response to signals in demyelinating lesions. To determine whether oligodendrocyte precursor cells exist in the adult human CNS, we have cultured white matter from patients undergoing partial temporal lobe resection for intractable epilepsy. These cultures contained a population of process-bearing cells that expressed antigens recognized by the O4 monoclonal antibody, but these cells did not express galactocerebroside (a marker for oligodendrocytes), glial fibrillary acidic protein (a marker for astrocytes), or vimentin. Selective elimination of O4-positive (O4+) cells by complement-mediated lysis resulted in inhibition of oligodendrocyte development in vitro. Since O4+ cells have an antigenic phenotype reminiscent of the rat adult oligodendrocyte-type 2 astrocyte progenitor and appear to develop into oligodendrocytes rather than type 2 astrocytes with time in culture, we call them “pre-oligodendrocytes.” Neither pre-oligodendrocytes nor oligodendrocytes incorporated 3H- thymidine in response to rat astrocyte-conditioned medium, platelet- derived growth factor, insulin-like growth factor (IGF-1), or basic fibroblast growth factor (bFGF). However, IGF-1 increased the relative abundance of oligodendrocytes to pre-oligodendrocytes, while bFGF had the opposite effect. Cells with the antigenic phenotype of pre- oligodendrocytes were also identified in tissue prints of adult human white matter. We propose that, in human demyelinating diseases such as multiple sclerosis, pre-oligodendrocytes may divide and/or migrate in response to signals present in demyelinated lesions and thus facilitate remyelination.