The distribution of twice-purified I-125-labeled alloantibody, prepared from the serum of strain DBA/2J mice obtained after immunization with strain C3H/HeJ spleen cells, was studied in immunosuppressed DBA/2J mice bearing either an allogeneic C3H/HeJ MCA sarcoma (i.e., one induced by 3-methylcholanthrene) or a syngeneic MCA sarcoma. Radiolabeled antibody was isolated from labeled immune gamma globulin by adsorption onto C3H/HeJ red blood cells and elution from stroma prepared from these cells, followed by Sephadex G-200 gel filtration chromatography. This purified antibody bound specifically in vitro to C3H/HeJ red blood cells. In vivo this antibody localized preferentially in C3H/HeJ MCA sarcomas. An enrichment procedure was developed to increase the yield of radiolabeled alloantibody. The localization of I-131-labeled alloantibody in allogeneic tumor was visualized by external scintigraphy. Control I-131-labeled normal DBA/2J gamma globulin did not show such tumor localization.