A clinical trial regimen modulating 5-fluorouracil (5-FU) with both folinic acid (FA) and recombinant alpha-2a-interferon (ralpha-2a-IFN) was noted to have a response rate of 54% and median survival of 16.3 months (Grem et al., J Clin Oncol 1993, 11: 1737-45). Reported herein is a phase II trial performed to further examine this regimen in metastatic colorectal cancer. Fifty-one patients with histologically proven, measurable advanced colorectal cancer with no prior therapy for metastatic disease were enrolled. ralpha-2a-IFN, 5 MIU/m2/day was given s.c. on days 1-7. FA, 500 mg/m2/day, and 5-FU, 370 mg/m2/day, were given i.v. on days 2-6. Cycles were repeated at 3 week intervals. Three complete and 12 partial responses were observed for an overall response rate of 29% (95% confidence interval: 18-45%). The median time to treatment failure and median survival were 4.6 and 15.5 months, respectively. Dose-limiting toxicities encountered were gastrointestinal, and included diarrhea, stomatitis, nausea and vomiting. These results do not support the concept of using concurrent ralpha-2a-IFN and FA as biochemical modulators of 5-FU. We observed increased toxicity and similar efficacy compared to using either modulator separately with 5-FU.