In this study, we investigated serum-soluble interleukin-2 receptor (sIL-2r) and neopterin (NPT) levels in five patients with severe postoperative infections. A total of 25 synchronous determinations of sIL-2r and NPT were performed. A marked increase in sIL-2r and NPT levels was observed, and the increase in sIL-2r was significantly correlated to that of NPT which is a marker of macrophage activity. These results suggest that macrophages are involved in the stimulation of sIL-2r release, representing a potentially negative biological effect. The results indicate that sIL-2r may be a useful indicator of the efficacy of antibiotics and of prognosis.