A comparison of the pharmacokinetics of intact B72.3 (a murine monoclonal antibody specific for human breast and colon carcinoma) with F(ab′)2 and Fab fragments labeled with 111In and 125I was done in athymic mice bearing target (LS174T) and non-target (HCT-15) tumors. IgG B72.3 labeled with either isotype imaged LS174T. Biodistributions of both labels were similar in all organs except liver. F(ab′)2 also imaged the LS174T tumor, while Fab bearing either isotope did not. The blood clearance was Fab > F(ab′)2 > immunoglobulin G B72.3 for both isotopes. 111In-labeled fragments yielded large accumulations in the kidneys which persisted for 2 days. The different patterns of biodistribution for the various forms of B72.3 labeled with the two isotopes suggest that the most desirable combination of fragment and isotope will depend on the intended use.