Sixty-five patients with advanced colorectal cancer were randomised to one of two schedules of recombinant alpha-2 interferon (IFN). In the first study, 36 patients received single-agent IFN, either 50 X 10(6) U/m2 intravenously on 5 consecutive days every 4 weeks, or 20 X 10(6) U/m2 subcutaneously three times per week. No tumour responses were seen and toxicity was unacceptable. In the second study, 29 patients received IFN in two similar schedules, but the dose of IFN was reduced to 20 X 10(6) U/m2 per day in the intravenous arm and to 5 X 10(6) U/m2 per day in the subcutaneous arm. In addition these patients were administered intravenous 5-Fluorouracil (5-FU), 250-500 mg/m2 per day on the first 5 days of each 4-weekly cycle. Although the toxicity of this second study was tolerable, only one short-lived partial remission was observed. Alpha-2 interferon, alone or in combination with 5-FU, is ineffective in advanced colorectal cancer.