AKR mice grafted with spontaneous AKR leukemia cells and treated with BCG may become leukemic due to graft leukemia or spontaneous leukemia, may die in a wasting-like syndrome, or may appear cured from leukemia and be without signs of wasting-like syndrome. Lymphoid spleen cells from these groups of mice were used at various times from cytotoxicity against AKR leukemia cells, against nonmalignant AKR fibroblasts, and against nonmalignant BALB/c fibroblasts. Spleen cells from cured mice and wasting-like mice were cytotoxic to AKR leukemia cells. The cytotoxic activity could be enhanced by depleting the lymphoid cells of T lymphocytes. AKR leukemia cytotoxic spleen cells were invariably also cytotoxic to AKR fibroblasts, but never to BALB/c fibroblasts. Quantitative differences in sensitivity to cytotoxic attack could be demonstrated between AKR leukemia cells and AKR fibroblasts since the former cell type was about 1000 times more sensitive to cytotoxic attack as compared to AKR-fibroblasts. The insensitivity of BALB/c fibroblasts to cytotoxic attack seemed to be a qualitative characteristic. Addition of lymphoid cells from cured AKR mice inhibited the cytotoxic activity of otherwise cytotoxic cells, thus indicating some suppressor activity in cured mice. It is suggested that successful anti-neoplastic immunoadjuvant treatment depends on a quantitative difference between malignant and nonmalignant cells with respect to cytotoxic attack of self-directed cytotoxic cells.