Abstract: C57BL/6J mice were given infusion of saline solution, syngeneic blood, allogeneic blood atfirst and 8th day. At 7th day after the second infusion, a single cell suspension of Bl6 melanomawas injected via the retro-orbital veins into C57BL/6J mice and both NK activity and proliferativeresponse of splenocytes were tested in vitro. The result showed that metastasis were significantlyincreased and both IVK activity and preliferative response of splenocytes were markedly depressedin allogeneic blood-transfused mice. There were no evidences of acceleration of tumor spread andsuppression of cellmediated immune response when those animals were transfused with syngeneicblood. It is suggested that the acceleration of metastasis seen following H-2-incompatible allo-geneic blood transfusions appears to be related to transfusion-induced immunosuppression.