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The low tumor-to-background ratio obtained after administration of radiolabeled whole monoclonal antibodies (MAbs) is one of the major problems in immunoscintigraphy and -therapy. To reduce the blood pool label caused by the circulation of radiolabeled MAb we have investigated the advantage of injecting an anti-antibody after administration of a tumor-specific MAb in nude mice bearing human mammary carcinoma xenografts. The MAb MA 10-11 of rat origin, used in these studies, had shown a high affinity to human mammary carcinoma tissue on frozen sections and low cross-reactivity with various normal human tissues. 24 h after injection of 1.5 MBq 131I-labeled MAb containing 10 micrograms IgG2a one group of mice received an additional injection of 100 micrograms anti-rat antibody. Blood taken 2 min after the second antibody injection showed nearly the whole activity bound to antibody aggregates, that cleared very rapidly from the circulation and accumulated in liver and spleen. The transitory high liver activity decreased within several hours because of rapid deiodination of the antibody-complex in this organ. The release of radioactivity from the spleen, however, was found to be much slower. The rapid excretion of the radioactivity from the blood pool combined with a nearly constant tumor activity allowed early tumor detection with tumor-to-blood ratios of 250:1 at 48 h after anti-antibody injection compared to 1.1:1 obtained for the control animals. In addition the results may explain the reported reduction of imaging quality and high uptake of radioactivity in the spleen of patients having repeated injections of mouse MAbs due to complex formation after development of human anti-mouse antibodies.