Oligodendrocytes isolated from 4-6-week-old rat brains were transplanted into newborn shiverer brains. Cells were identified as mature oligodendrocytes both by immunocytological and ultrastructural criteria. Normal myelin was detected using immunolocalisation (with an anti-MBP antiserum) and electron microscopy (presence of the major dense line). Patches of normal myelin (made by transplanted oligodendrocytes), widely spread throughout the host brains, were detected between 20 and 130 days after grafting. No sign of acute rejection was observed, but the graft became progressively delimited by astrocytic processes forming a continuous basal lamina.