The relative capacity of parental thymus and spleen cells to induce graft-versus-host reactions (GVHR), as reflected by spleen enlargement and stimulation of antibody formation, was studied in Fl hybrid mice. The relative GVHR-inducing abilities of the two cell types varied considerably, depending on the strain combination employed. The effect of parental lymphoid cells on spleen weight did not parallel their effect on antibody formation. In experiments using high concentrations of parental cells, thymus cells were more efficient than spleen cells as regards stimulation of antibody formation. The results indicate that thymus cells have a marked ability to induce GVHR. It is most unlikely that this ability is dependent upon the presence of contaminating peripheral lymphocytes in the thymus cell preparation used in the experiments.