A Phase II clinical trial of the combination of 5-fluorouracil (5-FU) and recombinant alpha-2a-interferon (alpha-2a-IFN) was conducted in 44 patients. Patients had not received chemotherapy previously and had measurable metastatic gastric carcinoma. 5-FU was administered as a continuous infusion at a dose of 750 mg/m2/d for 5 consecutive days and as an intravenous bolus at a dose of 750 mg/m2 weekly for 7 weeks beginning on day 12. Recombinant alpha-2a-IFN was administered subcutaneously at a dose of 9 x 10(6) U three times a week during weeks 1 to 8. Patients were examined for response during week 9. Of 44 patients entered, 40 could be examined for response. Nine patients experienced a partial clinical response and one achieved a complete response, for an overall response rate of 25% (95% confidence interval, 13% to 41%). The median duration of response was 13 weeks (range, 9 to 67 weeks) and the median survival time was 29 weeks. Grade 3 to 4 toxicities included granulocytopenia (nine patients), diarrhea (three patients), oral mucositis (seven patients), skin rash (one patient), and fatigue (four patients). One patient died of neutropenic sepsis. This regimen had modest activity with significant toxicity and produced responses of short duration. It did not appear to be superior to existing treatments of metastatic gastric carcinoma.