The serum levels of soluble interleukin-2 (sIL-2), sIL-2 receptors (sIL·2R), β2-microglobulin (β2M) and soluble intercellular adhesion molecule-1 (sICAM-1) were measured by the ELISA technique in 129 breast cancer patients and 40 controls. The median serum levels of sIL2-R, β2M and sICAM-1 were significantly higher and sIL-2 significantly lower than controls. sIL-2R, sICAM-1 and β2M levels were significantly higher in patients with metastatic disease compared to patients on long-term follow-up with no active disease. Initial study measurements of these markers could not identify patients at high risk for relapse. These findings suggest that the sIL-2R level is indicative of metastatic disease and together with other parameters of immune activation may be of help in monitoring disease activity in breast cancer pati