The postoperative survival rate of gastric cancer patients who received blood transfusions was found to be significantly lower than that of patients who did not receive transfusions in each of five postoperative years. When classified by stages, the five-year survival rate of stage I cases was significantly lower in the transfusion group. When the effect of blood transfusion on the antibodyproducing reaction was experimentally studied using mice, the antibody-producing ability was significantly suppressed 3 and 4 weeks after allogenic transfusion, but no suppression was observed in the syngenic transfusion group. This antibody-production inhibition of spleen cells was found to be attributable to Thy 1.2(+), Lyt-2(+) cells existing in the fraction of plastic petri dish non-adherent cells, that is, suppressor T cells. On the basis of these clinical and experimental findings, it was concluded that blood transfusion adversely affects the cancer-bearing patient and induces an immunosuppressive state through the antibody-production system.