In multiple sclerosis, partial remyelination is conspicuous in many lesions, but widespread and lasting myelin repair ultimately fails as disability and handicap accumulate. Thus far, the precise identity of the cell responsible for limited spontaneous myelin repair has remained obscure. In the rodent, the proliferative oligodendrocyte progenitor is the most efficient remyelinating cell; this has now been identified in cultures prepared from normal human brain, but has proved difficult to demonstrate in situ. We adapted techniques using antibodies against the human platelet-derived growth factor-alpha receptor to identify oligodendrocyte progenitors in human tissue sections. Small numbers of oligodendrocyte progenitors were found in normal adult human white matter. Progenitors were also demonstrable in acute and chronic lesions from patients dying with multiple sclerosis, but with no evidence of any marked reactive increase in cell numbers. Understanding the biology of the remyelinating cell, and in particular the reason for its apparent failure to repopulate demyelinated lesions, is important for the development of remyelination treatments.