Immunology, in all its facets of immunochemistry, humoral immunity, cellular immunity, immunodiagnosis, and immunotherapy, is currently the most rapidly advancing discipline of clinical and experimental oncology. But as in most developing subjects, it is continually beset with problems of methodology and nomenclature, both of which are often intertwined. Indicative of this problem are our circular definitions of antigen and antibody. Another fre quent problem is the use of the words "specific" and "associated" when characterizing re actions to tumors or the antigenic properties of neoplasms. When applied to cancer, these terms are obviously very dependent upon the methods used to demonstrate these qualities. Indeed, the very diverse nature and multitude of types of cancer preclude our making gen eralizations or categorical statements regarding tumor "specificity. " Similar problems arise when one considers that many antigens found in malignant tumors share certain properties with embryonic or fetal tissues. First, these observations emphasize a lack of true tumor specificity, for which reason the category of oncofetal antigens has been established. Second, whether or not such antigens are truly oncofetal depends upon the method of detection used, and it has been found that here too specificity decreases as the antigen assay increases in sen sitivity. Thus, whether substances can be truly oncofetal in nature still remains a matter of debate, and such terms must therefore be used in an operational sense.