The effects of tumor-BCG mixture to the cellular cytotoxicity of the regional lymph node and the spleen were observed in BALB/c mice received allogeneic tumor cells. This tumor cells, when grafted subcutaneously, were normally enhanced in 40% of BALB/c mice and 100% of A/St mice, but failed to grow when injected as a mixture with tumor and BCG. By injection of tumor-BCG mixture, the development and decline of the cytotoxicity((51)Cr release assay) to the tumor cells was accelerated, but the magnitude of the peak activity was diminished. Possible mechanisms by which the decreased activity induced of complete tumor suppression were examined. The result showed that immunized lymphocytes with non-immune macrophages exhibited a significatly increased cytotoxicity as compared with immunized lymphocytes in the abscence of macrophages.