ICRF-159, at a dose of 300 mg/m2, was given orally every 8 hours for nine doses every 21 days to 40 patients with metastatic breast cancer refractory to hormonal therapy and cyclophosphamide, methotrexate, 5-fluorouracil, and adriamycin chemotherapy. Two patients with soft tissue disease had short-lived partial responses. The hematologic toxicity was severe. Three patients required rbc transfusions. Four patients became septic at the nadir of leukopenia; two of these patients died while leukopenic. Two patients had platelet counts less than 25,000/mm3. All patients who were nonevaluable or who had life-threatening or lethal toxicity were nonambulatory. Since the 19 nonambulatory patients had a median survival of only 1.25 months as compared to 7 months in ambulatory patients, it is recommended that future phase II trials in chemotherapy-refractory breast cancer be limited to ambulatory patients. Although ICRF-159 has minimal antineoplastic effects, it is not recommended for further investigations in metastatic breast cancer, even at more hematologically tolerable doses of 250 mg/m2 every 8 hours for nine doses.