A graft-versus-host reaction was produced in (A X CBA) F1adult recipients following i.v. injection of 100 million CBA spleen cells. Subsequently a stronger response was evoked by presensitizing the donor animals, with or without sublethal irradiation of the hosts. The fate of the donor cells was studied by the T6 chromosome marker technique. Initial donor cell division in the spleen was observed in all the groups. It declined rapidly, except in irradiated recipients in which it continued till only donor mitoses were seen in the spleen and bone marrow from the 12th day to the end of the investigation on the 60th day, and subsequently on the 180th day. Signs of graft-versus-host disease when they occurred (usually in the 2nd and 3rd week) were associated with severe lymphoid depletion. In most cases rapid lymphoid division quickly replenished host lymphoid tissues and returned the animals to apparently normal health. In the present strain combination it did not appear to matter whether the repopulating lymphoid cells were host or donor in origin, providing the animal was not left depleted. Evidence was obtained that the transplanted donor cells became tolerant of host antigens.