Immunologic aspects of cancer have intrigued investigators for a long time. Much of the early enthusiasm for immunological control of cancer stemmed from laboratory experiments using transplantable tumors in animals. Unfortunately, many of these studies used tumors which were genetically foreign to the host and it was later apparent that much of the "cancer immunity" was merely an expression of genetic differences between the transplantable tumors and their hosts. In effect tumors were being used to study homograft responses. Following the advent of highly inbred strains of mice, it was possible to carry out such research with transplantable tumors in carefully controlled genetic systems. More recently spontaneous, carcinogen and virus-induced tumors have been widely studied. The voluminous literature which accumulated has been the subject of a number of excellent reviews among which those of Woglom, Hauschka, Snell and Gorer present distinctive viewpoints.'"' Much has been learned of the genetic and immunologic factors involved in -tumor transplant rejection. Variable degrees of resistance against transplantable tumors in animals may be obtained by active and passive immunization procedures. In some experimental systems involving genetically compatible host and tumors (as well as in certain carcinogen and virus-induced tumors) there is conclusive evidence that tumor specific antigens exist and that immunological reactions can alter the growth of such tumors. Among the many reports concerned with these studies are those of Amos et al; Gorer et al; Prehn, Habel, Sjogren et al; Sachs, Hirsch, Klein et al; Mirand and Grace."13At the clinical level there is evidence that "host factors" are associated with the development and progress of human malignant disease. Unfortunately most of this evidence is indirect, the mechanisms involved are poorly understood, and the role of immunological responses is obscure. The existence of "resistance" factors is suggested by such observations as (1) rare but authenticated cases of spontaneous regression of established tumors;'* (2) prolonged survival of patients after incomplete removal of their c a n~e r ;~~J~ (3) sudden appearance of widespread metastases many years after apparently successful treatment of the primary le~ion;'~JB (4) occasional regression of metastases after treatment of the primary tumor;14J6 ( 5 ) evidence that enormous numbers of the tumor cells are exfoliated into the blood stream and lymphatics without the development of a comparable number of ( 6 ) histological evidence of increased activity (histiocytic, lymphocytic and plasmacytic) in the area of some tumors and in the regional lymph nodes;21*22 (7) the appearance of antibodies against some components or products of their tumors in the serum of a few ~a t i e n t s ;~~ (8) the development of hypersensitivity by certain patients to some components or products of their t~m o r s ;~~~~ (9) demonstrable changes in immunological responsiveness paralleling exacerbations and remissions of certain malignant diseas...