Monoclonal antibody therapy may provide new treatment options in the management of metastatic breast cancer by selectively targeting tumors and producing a therapeutic effect, by delivering radiation or other toxins directly to tumor cells, or by producing an intrinsic immune inflammatory response. The effect of 131I-labeled humanized anti-Lewis(y) monoclonal antibody 3S193 (hu3S193) was compared with that of placebo and radiolabeled huA33 control antibody in a series of radioimmunotherapy experiments in a MCF-7 xenografted BALB/c nude mouse breast cancer model. The maximum tolerated dose of 131I-labeled antibody occurred at 200 microCi/mouse, at which dose level three of six mice that received 131I-hu3S193 showed significant tumor growth inhibition in contrast to no responses in the comparable 131I-huA33 control treatment arm. Breast cancer is an ideal model to test the efficacy of combined modalities given its known sensitivity to both radiotherapy and chemotherapy. The synergy between radioimmunotherapy and chemotherapy was therefore also explored using a combination of 131I-labeled hu3S193 antibody and Taxol using subtherapeutic doses of each agent. The combination of Taxol and 100 microCi of 131I-hu3S193 produced significant tumor inhibition in 80% of mice, whereas no responses were seen with either treatment modality alone or the combination of Taxol and 131I-huA33. These results support a potential therapeutic role of radiolabeled hu3S193 in the treatment of breast cancer, including combination therapy with Taxol, and warrants further investigation of this promising new agent.