Radioimmunotherapy (RIT) is a promising approach for treating metastatic breast cancer. Initial clinical trials using 131I radioimmunoconjugates, and more recent studies employing 90Y, have demonstrated objective, although transient, antitumor effects in heavily pretreated patients with minimal toxicity. Antibodies targeting unique epitopes of epithelial glycoprotein mucin (MUC-1) on breast cancer cell surfaces that have been studied in patients include BrE-3 (murine and humanized) and m170 (murine). Both antibodies react with at least 90% of breast cancers. In these and other RIT trials, myelosuppression has been the dose-limiting toxicity. However, this toxicity has been successfully circumvented with the use of autologous peripheral blood stem cell transplantation, and recent clinical trials have escalated 90Y doses up to 50 mCi/m(2). The therapeutic index indicates that using these agents with stem cell support should deliver 9000 to 18000 rads to metastatic tumors. Development of improved chelates that are readily metabolized in the liver may reduce doses to this organ, projected to be next in line as dose-limiting. Combination therapy will be required to produce durable benefits in metastatic breast cancer. Low dose taxanes are synergistic with RIT in preclinical studies and when administered in the optimal sequence could sensitize tumor cells to the continuous low dose radiation delivered by RIT, without increasing toxicity. The addition of systemically administered tumor targeting radiation therapy using RIT as part of combined modality therapy may enhance the rate of complete response and, in patients with minimal metastatic disease, could lead to curative therapy.