A possible consequence of an antilymphocytic autoimmune process would be serious impairment of an animal's ability to destroy tumor cells. One measure of autoimmune reactivity of this type would be the demonstration of cellular immune responsiveness by cells from tumor-bearing mice against syngeneic normal cells. These experiments demonstrate that spleen cells from mice bearing a lymphocytic leukemia of identical histocompatability type as the host mounted a vigorous immune response against normal syngeneic cells in a mixed lymphocyte reaction (MLR). Moreover, ascitic cells from leukemic mice responded significantly to normal syngeneic spleen cells in MLR's. The former reactions are usually much more vigorous than the responses of normal to malignant cells. These results are discussed in terms of the relationship between autoimmunity and neoplasia. Alternative explanations necessitated by the dangers involved in the interpretation of the immunology of transplantable tumors are considered.