Injections of lysolecithin were used to produce acute focal demyelination in the dorsal columns of 2 strains of mice, the myelin mutant quaking and the normal C57BL/6J. A small collection of rat Schwann cells grown in tissue culture was transplanted with their collagen substrate into this demyelinated area. The host mice were immune-suppressed to prevent graft rejection. Evidence of remyelination by Schwann cells was seen in the dorsal columns from 2-18 weeks after implantation. Proof that these Schwann cells were foreign to the host was derived from their rejection after the recipient mice were allowed to recover immunological competence by discontinuation of the immune suppression and by transferring immune cells sensitized against the donor tissue. It was concluded that Schwann cells grown in vitro retain their potential to produce myelin when returned to an in vivo situation and can myelinate central axons of a xenogenic host.