Biological response modifiers have been extensively investigated in the management of patients with cancer, but few data are available on tumors of the gastrointestinal tracts. To evaluate the feasibility and activity of the combination of interleukin 2 (IL-2) and β-interferon (β-IFN), 15 patients with colorectal cancer and 10 patients with hepatocellular carcinoma were treated with the following outpatient schedule: β-IFN 3 × 106 U i.m. 3 times weekly from day 1 to day 14; IL-2 4.5 × 106 U s.c. every 12 for 5 consecutive days a week from day 7 to day 21, the cycles being repeated every 28 days. All of the patients had been previously treated with chemotherapy; 7 unresectable locally advanced, and 18 had metastatic disease. All of the treatment courses were administered in an outpatient setting. No grade 4 side effects were recorded. The worst side effects were grade 3 fever (4 patients) and asthenia (2 patients) requiring treatment discontinuation in 4 cases. Twenty-four patients were evaluated for response (14 with colorectal cancer and 10 with hepatocellular carcinoma). One partial response, 4 stable disease, 9 progressive disease were recorded among the colorectal patients; 4 stable disease and 6 progressive disease among the hepatocellular carcinoma patients. The median duration of stable disease was 3 months for hepatocellular carcinoma and 4 months for colorectal cancer patients. Our results suggest that the schedule is feasible in an outpatient setting. Its limited hematological toxicity makes it suitable to be combined with cytotoxic d