Based upon prior data suggesting that alpha-interferon possesses chemomodulatory activity, a pilot study was conducted in which patients with advanced colorectal carcinoma were treated with 5-fluorouracil (5-FU), leucovorin (LV) and Roferon-A. Treatment consisted of LV 20 mg/m2 i.v. push followed by 5-FU, 425 mg/m2 i.v. push daily for 5 days every 4 weeks for 2 cycles, then every 5 weeks; Roferon-A 9 million units subcutaneously was given three times weekly every week. Forty-six eligible patients with bidimensionally measurable disease who had received no prior chemotherapy for advanced disease were treated with this regimen. The most frequent toxicity was leukopenia with 80% of patients experiencing some degree of leukopenia and the most severe toxicity was granulocytopenia with 46% of patients experiencing granulocyte counts < 1,000/mm3. Among the 46 eligible patients, the objective response rate was 13% (95% confidence interval, 5-26%). Thirty-five of the 46 patients have died with a median survival of 17 months. This regimen has significant toxicity and insufficient activity against advanced colorectal carcinoma to warrant further trials.