Mc3 is a murine mAb that is highly effective in treating breast tumors in experimental radioimmunotherapy. Mc3 binds to BA46, a 46-kDa glycoprotein of the human milk fat globule membrane that is also expressed in breast carcinoma cells. We cloned and sequenced cDNAs encoding the variable regions of Mc3 and constructed an IgG1, kappa human/mouse chimeric antibody. We then humanized the variable regions of Mc3 using a positional consensus method and retaining residues that might either contact the complementarity-determining regions or the opposite chain. This positional consensus is novel in that it does not include residues with buried side chains. Humanized Mc3 retained full binding affinity, and fully competes with murine Mc3 for antigen binding. Humanized and murine 131I-labeled Mc3 behaved identically in athymic nu/nu mice biodistribution studies. The tumor uptake levels for both antibodies increased over a period of 4 days within a range of 13-20% of the injected dose per g with extremely favorable tumor:normal ratios. Also, a single therapeutic dose of 131I-labeled humanized Mc3 in the same animal model reduced the average tumor size and produced one of five cures while in the uninjected control tumor growth continued unabated. We believe that these results justify the implementation of Phase I human clinical trials for imaging and radioimmunotherapy of breast cancer.