Background: A phase II study was performed to evaluate the clinical and immunological effects of a regimen of fluorouracil (5-FU) and folinic acid (FA) combined with thymopentin (TP-5) and interleukin-2 (IL-2) in the treatment of patients with metastatic colorectal cancer. Patients and methods: Forty-five evaluable patients with measurable colorectal cancer and no prior therapy for metastatic disease were treated with 5-FU 400 mg/m(2)/d and FA 200 mg/m(2)/d i.v. on days 1-5, TP-5 50 mg s.c. on days 8-11, and IL-2 9 MLT/m(2) s.c. twice daily on days 12-16. Cycles were repeated at 4-week intervals if toxicity had resolved. Immunological changes were evaluated in 13 patients and compared with a well matched series of 13 patients treated with the same regimen without TP-5. Results: Two complete responses and 17 partial responses were seen (42%; 95% confidence interval, 28% to 56%). Fifteen patients (33%) had stable disease. The median time to progression was 8.5 months and the median survival 13 months. Treatment was reasonably well tolerated, and there was no overlapping toxicity or interference between chemotherapy and biotherapy. Hematological and immunological changes during treatment were qualitatively similar to those expected with IL-2+/-chemotherapy. Quantitatively, significant changes (higher levels of IL-2, CD25 and IFN-gamma, and lower levels of sIL-2R) were observed in patients given TP-5. Conclusion. The combination of 5-FU + FA and TP-5 + IL-2 is effective in advanced colorectal cancer with accept able toxicity. Immunological data suggest that TP-5 may modulate the action of IL-2 in the clinical setting. However, improved treatment approaches are needed, and the interactions between thymic hormones and cytokines should be further explored.