In these studies adult mice treated with cyclophosphamide and foreign immunologically competent cells developed a graft versus host disease which outwardly resembled that encountered in other experimental systems. Progressively larger doses of cyclophosphamide produced an increasingly severe disease whereas comparable doses of mechlorethamine were ineffective. Increasingly larger cell inocula from parental, allogeneic, and xenogeneic donors resulted in a correspondingly more severe disease. Nucleated cells obtained from the peripheral blood were found to be the most potent inducers of this syndrome, while cells from the spleen, bone marrow, and thymus displayed lesser degrees of reactivity in that order. No such graft versus host disease occurred in mice given saline, lysed, or heat-killed cells in place of viable foreign cells. Neither did the disorder develop when comparable inocula of isogeneic cells were used.