SUMMARYA model of the graft-versus-host reaction (GVHR) was produced in BALB/c newborn mice by injection of 6 X 10° viable adult C57BL spleen cells i.p. within the first 24 hr of life. The mortality from the GVHR was significantly modified by the use of heterologous antilymphocyte serum produced in rabbits. Early and prolonged treatment increased the effectiveness of the antilymphocyte serum. Donor cells could be demonstrated in about two-thirds of the survivors in the treated animals after cessation of therapy.