Immune unresponsiveness to HSA and BSA was induced in adult rabbits exposed to 550 R. total-body irradiation. Immunogenic doses of antigen, 20–75 mg., were found to be tolerogenic in this system. The degree of unresponsiveness was found to be a function of the time interval between X-irradiation and the beginning of antigen administration. The unresponsive animals retained the circulating antigen in quantities detectable by the gel-diffusion technique. These results were compared with the state of unresponsiveness induced by means of 6-mercaptopurine, and similar parameters were found to be operative. Some aspects of the significance of the ratio of antigen to competent cells, in the induction of immune unresponsiveness, are discussed.