Murine IgG2a monoclonal antibody (MAb) 425 specifically detects epidermal growth factor receptor, which is expressed on human gliomas and tumors of other tissue origin but rarely on normal brain tissues, and not at all on bone marrow and peripheral blood cells. 131I-labeled F(ab')2 fragments of this MAb injected into nude mice grafted with U-87 MG glioma cells preferentially localized in tumor tissue compared to normal mouse tissues, as determined by differential tissue counting of radioactivity. The mean tumor-to-tissue ratios of radioactivity ranged between 8.2 (blood) and 55.8 (muscle) at 2 days after the injection of 15 muCi of 131I-425 F(ab')2/mouse. Radiolabeled fragments of an anti-hepatitis virus IgG2a MAb did not localize in tumors. The localization index derived from the ratios of specific antibody to indifferent antibody in tumor tissue relative to blood was 9.94 at 2 days following the MAb injection. The labeled MAb did not localize in a xenograft of colorectal cancer tumor, which does not express the epidermal growth factor receptor. Tumors could be located by whole-body gamma-scintigraphy without background subtraction following the injection of 100 muCi of radiolabeled MAb 425 F(ab')2 fragments. The data suggest that MAb 425 is a likely candidate for clinical diagnostic and radioimmunotherapy trials.