One form of the present invention is directed to a method of remyelinating demyelinated axons by treating the demyelinated axons with oligodendrocyte progenitor cells under conditions which permit remyelination of the axons. Another aspect of the present invention relates to a method of treating a subject having a condition mediated by a loss of myelin or a loss of oligodendrocytes by administering to the subject oligodendrocyte progenitor cells under conditions effective to treat the condition mediated by a loss of myelin or a loss of oligodendrocytes. A further aspect of the present invention relates to an in vitro method of identifying and separating oligodendrocyte progenitor cells from a mixed population containing other mammalian brain or spinal cord cell types. This further aspect of the present invention involves removing neurons and neuronal progenitor cells from the mixed population to produce a treated mixed population. Oligodendrocyte progenitor cells are then separated from the treated mixed population to form an enriched population of oligodendrocyte progenitor cells.