To determine the importance of the label in the radioimmunoimaging of lesions, we studied the distribution of a monoclonal antibody (103D2) that had been labeled with both 125I and 111In. Antibody 103D2 is specific for a tumor-associated, 126-kd phosphoglycoprotein. We used the dual-labeled antibody to localize BT-20 human mammary tumors hosted in nude mice (n = 20). The means of the ratios of 111In to 125I activity in the blood, heart, lungs, liver, spleen, kidneys, thyroid, and tumors were compared at 1, 2, 3, 5, and 6 days after the i.v. administration of antibody. The mean (+/- s.d.) of the ratios of 111In to 125I activity in the tumor was 2.3 +/- 0.8 at Day 1, which increased to 9.6 +/- 1.7 by Day 5 and 12.7 +/- 5.5 by Day 6, whereas the mean of the ratios in the blood was 1.1 +/- 0.3 at Day 1, 0.9 +/- 0.2 at Day 5 and 2.0 +/- 1.0 by Day 6. These results suggest that the circulating dual-labeled antibody remains intact, but that dehalogenation occurs at the tumor site.