There are a few reports about the effect of splenectomy done after tumor transplantation on tumor growth and cell-mediated immunity. In this study, splenectomy was performed in PDS mice transplanted s.c. with Erlich ascites tumor cells before and after the transplantation to clarify the significance of splenectomy in tumor-bearing bodies. The tumor size and macrophage migration inhibitory activity (MIF) were measured as parameters of cell-mediated immunity with the lapse of days after the transplantation. 1. Effect of splenectomy on tumor growth and the survival period of mice transplanted with 5×10(6) tumor cells/mouse. (1) In the first group undergoing sham operation 7 days before transplantation, all mice died of tumor within 30-50 days after the transplantation. (2) In the second group undergoing splenectomy 7 days before transplantation, the tumor propagated temporarily then regressed completely in each mouse. (3) In the third group that underwent splenectomy 5 days after transplantation when the tumor had become palpable, the tumor regressed completely in 85% of mice. The rest of the mice died of growing tumor. (4) In the fourth group undergoing operation 10 days after transplantation, all mice died of the rapidly growing tumor and died earlier than the first group. 2. Effect of splenectomy on tumor growth and the survival period of mice transplanted with 5×10(5) tumor cells/mouse. (1) In the fifth group undergoing sham operation 7 days before transplantation, all mice died of tumor similar to the first group. (2) In the sixth group undergoing splenectomy 10 days after transplantation, 40% of mice died of tumor growth similar to the fifth group. The remaining 44% of mice survived 2 times longer than the fifth group and the tumor regressed completely in 16% of the mice. 3. Effect of splenectomy on the MIF of regional axillary lymph node cells. In every group, the MIF has a close relationship with the tumor growth. The MIF reached its maximum level on about 5 days after transplantation by which time the tumor had become palpable in the first and the fifth group. Thereafter, MIF decreased and had disappeared by 2 weeks after transplantation. In the mice in which the tumor regressed completely, a weak MIF was found during tumor growth and this was still moderate 2 weeks after the complete tumor regression. Splenectomy seems to be useful in the relatively early cancer as it inhibits relapse after the radical operation.