We have recently reported high clinical activity against advanced colorectal cancer of a regimen-alternating bolus FUra, modulated by methotrexate (MTX), and continuous infusion FUra, modulated by 6-s-leucovorin (6-s-LV). Considering the low toxicity of the bolus part of this regimen and our recent in vitro finding of a strong synergism between bolus FUra and natural-beta-IFN (n-beta-IFN), this cytokine was incorporated in the bolus part of our treatment programme. Fifty-six patients with untreated, advanced, measurable colorectal cancer were treated with two biweekly cycles of FUra bolus (600 mg m(-2)), modulated by MTX (24 h earlier, 200 mg m(-2)), and n-beta-IFN (3 x 10(6) IU i.m. every 12 h, starting at the time of FUra administration for four doses), alternating with a 3-week continuous infusion of FUra (200 mg m(-2) daily), modulated by 6-s-LV (20 mg m(-2) weekly bolus). After a 1-week rest, the whole cycle (8 weeks) was repeated if indicated. A total of 5 complete and 17 partial responses were obtained (response rate, 41%; 95% confidence limits, 28-55%) in 54 assessable patients. After a median follow-up time of 36 months, five patients are still alive. Overall, the median time to treatment failure was 6.4 months. The median duration of survival was 15.0 months. There was one treatment-related death after a course of MTX --> bolus FUra/n-beta-IFN and grade III-IV toxicity occurred in 18% of the patients. As the addition of n-beta-IFN results in high toxicity, whereas the efficacy seems to be similar to that of the same regimen without the cytokine, our groups are currently randomizing the original regimen, without IFN, against standard modulated bolus FUra.