This study was directed toward determining the pharmacokinetic fate of an IgG2a monoclonal antibody (MoAb). The 96.5 anti-melanoma MoAb was labeled with indium-111 and indium-114m and administered to BALB/c mice. The mice receiving 111In MoAb were sacrificed at 4 and 72 h, while those receiving 114mIn 96.5 MoAb (50-day physical half-life) were sacrificed at 4 h and 3, 15, and 30 days. Multiple tissues were counted against a standard of the injectate and the data expressed as percent injected dose per organ and percent total dose excreted in the urine and feces. The 111In- and 114mIn-labeled MoAbs had nearly identical distribution through 72 h. Over the 30-day period 25% of the 114mIn label was excreted in the urine and 50% eliminated in the feces. All of the tissues studied showed a decrease in 114mIn in the 30-day period. We conclude that the metabolic products of indium-labeled MoAbs, the indium itself, or a combination of both are eliminated from the tissues over a period of several weeks and do not accumulate to a significant extent in any single site.