C-reactive protein (CRP) levels in serum were measured in fifteen patients with metastatic colorectal carcinoma, prior to and during treatment with a continuous intravenous infusion of rIL.2. Patients were subsequently classified as responders or non-responders to this therapy. Baseline serum CRP levels, prior to treatment, were significantly lower in the responders (range < 2-8 mg l-1) when compared with the non-responders (range 7.5-116 mg l-1), P = 0.004. Furthermore, the responding patients demonstrated significantly and grossly elevated CRP stimulation indices (SI) compared with non-responders at different time intervals during the rIL2 infusion. At the cessation of rIL2 therapy, the CRP stimulation index was 31.3 +/- 9.3 in the responders, and only 1.6 +/- 0.3 in the non-responders (means +/- s.e.m, P = 0.014). These findings suggest that it is possible to predict those cancer patients who are most likely to respond to and benefit from rIL2 therapy, either prior to the commencement of or during the first course of rIL2.