Neonatally thymectomized (A X C57BL/1)F1hybrids grafted with thymus from parental strain A donors developed evidence of graft-versus-host reactions, including weight loss and poor survival. Syngeneic spleen cells did not reverse this wasting syndrome. Immunological restoration was, however, observed in the surviving animals. When the neonatally thymectomized (A X C57BL/1)F1mice were grafted with syngeneic hybrid thymus no signs of wasting were observed and the animals were reconstituted immunologically. The thymectomized untreated animals showed a low incidence of wasting syndrome, which was readily reversed with syngeneic spleen cells. Such animals were unable to reject an allogeneic tumor graft but could reject an allogeneic skin graft with almost normal vigor. The results obtained after grafting of parental strain thymus into thymectomized hybrids suggest that a population of cells derived from the thymus plays an important role in expression of the graft-versus-host reactions observed.