Thirty-five patients with metastatic breast cancer who had received one or two prior chemotherapeutic regimens were treated with elliptinium acetate at a dose of 80 mg/m2 for 3 days every 3 weeks. Of the 33 patients evaluable for response, 1 patient achieved complete remission, 4 achieved partial responses (15% overall objective response with 95% confidence interval of 5–32%), and 6 achieved minor response. Toxicity of the treatment was xerostomia, diarrhea, and nausea and vomiting. The drug was not myelosuppressive. Three patients showed evidence of elliptinium antibody, and treatment was discontinued. No episodes of hemolysis were observed. Elliptinium acetate showed modest antitumor activity in previously treated patients with metastatic breast cancer.