In 67 patients submitted to surgical procedures serum neopterin (NPT) and Interleukin 2 soluble receptors (IL2R) were evaluated at the end of the operation as well as 24, 48, 72 hours later. Thirty seven of the subjects (Group B) had undergone minor surgery (average time of operation: 40 +/- 10 min.), thirty (Group A) had undergone major surgery (average time: 180 +/- 30 min.). The results showed elevated NPT and IL2R levels in the latter cases and, in particular, 48 and 72 h after surgery. Neopterin levels were positively correlated with IL2R (r = 0.548 p < 0.01). These data suggest an activation of the cellular immune response which parallels the magnitude and length of surgical trauma. Thus NPT and IL2R levels could represent biochemical markers of postoperative disorders of the immune homeostasis.