Sixty-three patients with Stage IV breast carcinoma refractory to standard combination chemotherapy agents such as 5-fluorouracil (5-FU) were entered into a study to determine the efficacy of a multiple dose schedule of N-(phosphonacetyl)-L-aspartic acid (PALA) and whether the addition of PALA improves the therapeutic efficacy of 5-FU. Patients were randomized to receive either PALA, 800 to 1000 mg/m2 per day for 5 days every 2 weeks; or PALA + 5-FU, 400 mg/m2 per day for 5 days, and 300 mg/m2 per day for 5 days every 28 days, respectively. The PALA alone arm of the study was closed after 20 patients had been treated and was replaced by 5-FU, 300 to 400 mg/m2 per day for 5 days every 21 days. Overall response rates were 5% for PALA alone, 28% for PALA + 5-FU, and 14% for 5-FU alone. All patients who responded to PALA + 5-FU or 5-FU alone had received prior therapy in which 5-FU was part of the combination chemotherapy program and were considered refractory to this drug. Toxicity affected the gastrointestinal tract but was tolerable in all three arms of the study. Myelosuppression was negligible for PALA and PALA + 5-FU and moderate for 5-FU. The authors concluded that PALA + 5-FU was superior to PALA alone in the therapy of these heavily pretreated patients. PALA alone had marginal efficacy. In view of its low hematologic toxicity, PALA + 5-FU may be combined with more myelosuppressive drugs. Additional studies are necessary to ascertain whether PALA + 5-FU is therapeutically superior to a full-dose schedule of 5-FU.