The combination of 5-fluorouracil (5-FU) and interferon-alpha (IFN-alpha) has reported activity in the treatment of advanced colorectal carcinoma. Laboratory studies of IFN-beta suggest that this agent may offer theoretical advantages over IFN-alpha in combination with 5-FU. A total of 27 patients with advanced or recurrent colorectal carcinoma were treated in a non-randomized open phase II study with a combination of 5-fluorouracil (750 mg m(-1) daily for 5 days as a continuous intravenous (i.v.) infusion followed, from day 15, by i.v. bolus 750 mg m(-2) every 7 days) and recombinant interferon-beta [r-hIFN-beta-1a; 9 MIU (total dose) by subcutaneous injection from day 1 on every Monday, Wednesday and Friday throughout the treatment period]. Toxicity was less than that seen with this schedule of 5-FU in combination with IFN-alpha. Among 21 evaluable patients, four objective responses were seen. Recombinant human interferon-beta-1a in combination with 5-FU is an acceptable regimen in terms of toxicity. However, the study did not demonstrate a superior response rate when compared with previous reports of treatment with 5-FU alone or in combination with IFN-alpha.