Twenty-four patients with metastatic colorectal cancer were treated with recombinant IL-2 (rIL-2) by continuous intravenous infusion for 5 days (18 x 10(6) U/m2 per 24 h), followed by three injections of 5-fluorouracil (600 mg/m2) and folinic acid (25 mg/m2) at weekly intervals. The response to treatment was assessed using standard UICC criteria (partial or complete response, stasis or progression of disease). The serum concentrations of the acute phase proteins; C-reactive protein (CRP), retinol binding protein (RBP), alpha 1-antitrypsin (alpha 1-AT), transferrin (TF) and albumin were measured. A response to therapy occurred in the tumours of seven (29%) of the 24 patients (two complete and five partial responses). All patients who demonstrated a response to treatment had a serum albumin level of > 37 g/l and a CRP level of < or = 10 mg/l. In contrast, of the 17 patients who did not respond to therapy, 12 (71%) had a serum albumin of less than 37 g/dl and a CRP of greater than 10 mg/l. Examination of the survival times of the 12 patients who had a pretreatment serum albumin level of less than 37 g/l revealed that all had died within 12 months of cessation of therapy. However, 58% of patients with pretreatment serum albumin levels of greater than 37 g/l survived for longer than 12 months. These results have shown that (i) patients who respond to rIL-2-based therapy and (ii) those patients who have prolonged survival times, can be identified by pretreatment measurement of serum levels of acute phase proteins.