The effect of anti-host antiserum was studied upon the course of graft-versus-host disease induced by injecting parental spleen cells into adult F1 hybrid mice. Two types of result were observed: (1) antibody acted synergically with parental spleen cells, potentiating their effect, and thus curtailing the survival times of the recipient F1 hosts; (2) alternatively, antibody interfered with the action of parental spleen cells, and prolonged the survival times of the F1 recipients. Serological analysis of surviving (A ± C57BL)F1 hosts previously injected with C57BL spleen cells and C57BL anti-A serum showed that donor type cells persisted for periods longer than 3 months.