Tumor immunology as an accepted discipline is only some 20 years old, although there has been a lively interest in the possibility of conquering cancer by the body's own defense mechanism since the beginning of the century. Unfortunately, many of the successes reported by early workers with transplanted tumors have been discredited on the basis of histocompatibility differences. The experimental animals may not have been inbred; thus, tumor rejection was more likely due to the normal histocompatibility antigens than to specific tumor antigens. Therefore, the important works by Gross (1943), Foley (1953), and Gorer (1956) met a skeptical audience, until R. T. Prehn and J. M. Main (1957) showed beyond reasonable doubt, in a strictly genetically controlled work, that the tumors can indeed elicit an immune response against themselves, i.e., that they can be recognized as nonself. Now experimental tumor immunology has expanded greatly. Largely by work in vitro, which used sophisticated immunological methods, the knowledge of the workings of the immune system has grown and become very complicated in the process. As in any biological system, the immune system is also full of checks and balances, feedbacks and redundancies. There are initiators, helpers, suppressors, killers, and natural killers among the T-cells (thymus-derived lymphocytes) alone. Although the B-cells (bone marrow-derived lymphocytes) are mainly charged with making antibodies, they also perform some of the functions