This review covers a number of aspects of the behaviour of oligodendrocyte progenitors following transplantation into the adult CNS. First, an account is given of the ability of transplanted oligodendrocyte progenitors, grown in tissue culture in the presence of PDGF and bFGF, to extensively remyelinate focal areas of persistent demyelination. Secondly, we describe how transplanted clonal cell lines of oligodendrocyte progenitors will differentiate into astrocytes as well oligodendrocytes following transplantation into pathological environments in which both oligodendrocytes and astrocytes are absent, thereby manifesting the bipotentially demonstrable in vitro but not during development. Finally, a series of studies examining the migratory behaviour of transplanted oligodendrocyte progenitors (modelled using the oliodendrocyte progenitor cell line CG4) are described. These show that CG4 cells do not survive (or migrate) when transplanted into the normal adult CNS. However, if they are transplanted into CNS tissue that has previously been exposed to 40 Gy of x-irradiation then transplanted CG4 cells survive, divide and migrate over large distances. Moreover, within an x-irradiated environment, migrating transplanted CG4 cells are able to enter remotely located foci of demyelination and contribute to the remyelination of the demyelinated axons within. These studies demonstrate that although the normal adult CNS does not appear to support survival and migration of the CG4 cell line, it is possible to manipulate the environment in such a way that these nonpermissive properties of the environment can be overcome.