We have employed noninvasive, external counting techniques for quantitation of I-131 F(ab')2 fragment of mouse monoclonal antibody localized in metastatic lesions and surrounding liver tissue in humans. This method utilizes counts from diametrically opposed views of tumor deposits and surrounding normal tissue. Corrections were made for patient attenuation, lesion size, and surrounding tissue activity. The validity of this method was evaluated using a fillable, tissue-equivalent organ-scanning phantom with organs and tumors of selected size. Less than 10% error was found in quantitation of various activities of I-131 in a 4-cm-diam lesion. Tumor activity ranged from 0.001% to 0.018% of administered dose per cm3 of tissue compared with 0.000 12% to 0.0023% per cm3 of liver. In addition, the vascular clearance of total I-131 and protein-bound I-131 was found to follow a two-compartment model with mean half lives of 3.8 and 21.4 h for total I-131 and 3.9 and 24.4 h for protein-bound I-131.