1) It has been demonstrated that the delayed type allergy induced by tuberculin in cancer bearing mice is weakened along with the progress of cancer growth. 2) Pedigree mice can be made tuberculin reaction positive by BCG sensitization. 3) After transplanting Ehrlich tumor to the mice made sensitive to tuberculin by BCG injection and to the control group injected only with abjuvant, the tumor growth in these two groups of animals was followed up with lapse of time. As the result it was found that in the tuberculin positive mice the tumor was seen enlarged 2-3 weeks after the transplantation but thereafter it grew smaller. 4) Using the spleen cells obtained from the mice positive to tuberculin and the controls injected only with adjuvant, the tissue cultures were conducted by mixing the spleen cells with JTC-11 culture cells derived from Ehrlich cancer in order to compare the anti-tumor activity. As the result it was demonstrated that in the BCG positive group even when the tumor has shrunken their spleen cells retain the anti-tumor activity, and the lymphoid cells from the spleen of the tumor bearing mice (after transplantation of Ehrlich tumor) show anti-tumor activity to JTC-11 cells in mixed cell culture and this anti-tumor activity seems to be somehow associated with the extent of tumor growth.