Remyelination of ethidium bromide induced areas of demyelination in the adult rat spinal cord is normally carried out by Schwann cells. When CNS cultures containing large numbers of oligodendrocytes, oligodendrocyte precursors and type-1 astrocytes were injected into such lesions 3 days after the injection of ethidium bromide, remyelination was carried out by oligodendrocytes. When cultures deficient in type-1 astrocytes, prepared by shaking off and subculturing top-dwelling cells, were used there was only a modest increase in the extent of oligodendrocyte remyelination over that seen in uninjected lesions; the majority of axons being remyelinated by Schwann cells. To prove that these Schwann cells were mainly locally derived, shaken cultures were injected into lesions prepared in areas of the spinal cord locally X-irradiated with 40 Grays to inhibit host repair. In these animals the extent of oligodendrocyte remyelination achieved was similar to that seen when unshaken cultures (rich in type-1 astrocytes) were injected into lesions made in non-irradiated tissue. These results indicate that type-1 astrocytes control Schwann cell remyelination of CNS axons.