Fifty-three patients with metastatic breast carcinoma were randomized to treatment with hexamethylmelamine (HMM) as a single agent versus a three-drug reimen of HMM, vincristine, and mitomycin C (HOM). All patients had received prior treatment with 5-fluorouracil, Adriamycin, and cyclophosphamide with or without methotrexate. HMM alone was used in a dose of 300 mg/m2/day x 14 days every 21 days. In the HOM regimen, the HMM dose was 200 mg/m2/day x 21 days, the vincristine dose was 1.5 mg on Days 1, 8, and 15, and the mitomycin C dose was 12 mg/m2 once every 6 weeks. No objective responses were observed with HMM in 15 evaluable patients. The HOM regimen resulted in five partial responses among the 27 evaluable patients. Gastrointestinal toxicity was the limiting toxicity of HMM, and thrombocytopenia was the major toxicity of the HOM regimen.