The problem of natural and acquired immunity to cancer has been extensively studied during the past fifty years. It is an established fact that such an immunity exists against transplanted animal tumors (2, 4, 8, 12, 14), although its existence in the case of spontaneous tumors is more difficult to determine. Studies in the heredity of cancer would indicate that natural immunity occurs in both animals and man. Wood (25) has stated, however, that there is no evidence that regression of spontaneous tumors in man, at least, depends upon any process of acquired immunity. The mode of the production of natural and acquired immunity to transplantable tumors and their metastases in experimental animals has not been determined. Ewing (10), in 1922, called attention to the influence of the tumor bed in the establishment of metastases. Loeb (18) thought the local tissue defense against cancer was essentially a foreign-body reaction. Growth-inhibiting factors in normal serum have been postulated by Carrel (6). Hektoen (13) believed antibodies against foreign cells were formed in the bone marrow. It has never been possible, however, to transfer immunity from one animal to another by any method using serum or normal cells or their products (2, 12, 15). Several writers have believed the defense against cancer a function of the so-called “reticulo-endothelial system.” In 1932 Foulds (11) indicated the protective rôle of these cells in preventing splenic metastases in animals inoculated with the Brown-Pearce rabbit carcinoma. Arons and Sokoloff (1) in a recent paper reviewed the literature on this subject, adding material, both experimental and clinical, to support their contention that susceptibility to cancer can be altered by stimulating or depressing the reticulo-endothelial cells throughout the body. The existence of a circulating cellular defense against neoplastic tissue has been the subject of much debate. Kardjiev (14) in 1937 described an overwhelming lympho-cytic reaction about abortive metastases in the lungs of rabbits immunized against the Brown-Pearce tumor. Previously, Murphy and his co-workers (16, 17), Russ and collaborators (21), and Clarkson and May-neord (9), all had developed experiments indicating that the circulating lymphocyte played a definite role in the immune reaction. This view, however, was opposed by Prime (20) and Sittenfield (24), who were unable to duplicate the experiments which led to it. Brown and Pearce (3, 4, 5) and Casey (8) working with the Brown-Pearce rabbit carcinoma were content to ascribe immunity to some constitutional factor as yet unknown. The phenomenon of the resistance of certain organs to metastases is an important part of the general concept of cancer immunity. It is well known that in most animals, and in man, the spleen is rarely involved by secondary epithelial neoplasms (11). Conversely, in certain tumors the predilection of metastases for special organs has become a pathological commonplace.