Humoral responses in tumor immunology have been reviewed in light of past and recent experimental findings. The basic concepts of tumor immunity, both cellular and humoral, did not emerge until tumor-specific transplantation antigens were distinguished from histocompatibility transplantation antigens. Experimental findings have demonstrated the presence of tumor-specific and/or associated antigens in both chemically induced as well as spontaneously arising tumors. The antigens resulting from chemical induction have been found to possess individual specificity while those from spontaneous tumors displayed cross-reacting antigenic reactivity. Cellular and humoral factors are both inherent in the framework of tumor immunology. Cell-mediated tumor immunity has been only briefly reviewed in this report; emphasis was primarily focused on the humoral aspects. Humoral tumor immunity has been discussed on the basis of the biochemical nature and immunobiological activity of the circulating substances known to date. The use of radiolabeled antibodies in tumor immunology has also been presented from the standpoint of tumor localization and potential therapeutic use. Finally, the utilization of the radioimmunoassay in tumor therapy and diagnostics has been reviewed.