During the past decade it has become increasingly apparent that cancer cells have acquired, during neoplastic transformation, new antigens not found in normal cells and that the host's immune response against these antigens is important in controlling the development and progression of malignancy. In immunologic and immunotherapeutic studies with human melanomas and sarcomas a remarkable correlation between the patient's immune response to his tumor and the extent and progression of malignant disease was found. Rising titers of antitumor antibody were consistently observed after surgical removal of the tumor mass in patients with sarcomas, whereas all patients with recurrent disease were found to have a progressive decline in their titers of antitumor antibody with advancing disease. Immunotherapy studies in guinea pigs and man have shown that active immunization with BCG vaccine and autologous tumor cells induced a heightened immune response against the tumor-associated antigens that was sometimes therapeutic.