The allogeneic effect was used as a potent immunopotentiator in preventing the growth of a murine plasmacytoma and prolonging host survival. Parental BALB/c spleen cells were passively transferred to (BALB/c .times. A/J)F1 hybrid mice, who were then given a highly lethal dose of MOPC 315 plasmacytoma, a tumor of BALB/c origin. The resultant graft-vs.-host reaction protected the recipient mice against growth of the tumor and significantly prolonged survival. This phenomenon was dependent upon the dose of BALB/c lymphoid cells employed, the route of administration, and the time interval between lymphoid cell transfer and tumor inoculation. A wide range of lymphoid cell doses and time intervals were effective, and repeated doses of allogeneic cells provided better protection than a single dose.