The eosinophil activity in patients with renal cell adenocarcinoma during treatment with interleukin-2 (IL-2) and interferon-alpha (IFN-alpha) was reduced when measured as zymosan-induced lucigenin-enhanced chemiluminescence (CL). Eosinophil cationic protein (ECP), eosinophil peroxidase (EPO) and eosinophil protein-X (EPX) were significantly elevated before and during treatment (P<0.001) when compared with the controls. ECP and EPO were unaffected by the treatment whereas it induced an increased EPX level compared with values measured before treatment (P<0.05). The propensity of eosinophils to secrete their granule proteins may reflect the fact that eosinophils in cancer patients have an enhanced capacity to kill cancer cells.