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Summary The effect of donor or host immunologically competent cells added 21 days after lethal irradiation and allogeneic bone marrow transplantation was investigated in mice. Whereas 10 × 106 normal host lymphocytes or spleen cells had no effect on the subsequent course of secondary disease, 50 × 106 normal host spleen cells significantly improved survival. Normal donor cells were efficacious at both dosages, but particularly at the highest inoculum. Cells obtained from strains unrelated to the donor or host were either not effective or harmful. To test the susceptibility of the chimera 21 days after bone marrow transplantations, spleen cells preimmunized to either donor or host tissues were administered and found to be consistently destructive. The mechanism whereby large innocula of normal donor or host cells improve survival is unknown, but a hypothesis based on inhibition of the primary immune response by serum antibody if offered for consideration.