The correlation between state of preoperative immunity assessed by in vitro test and the onset of postoperative septic complications has been evaluated. 153 patients subjected to surgery of choice have been examined. Before the operation cutaneous delayed hypersensitivity tests with recall antigens (streptokinase-streptodornase, purified tuberculoprotein and candidine) were carried out. 111 patients were classed as immunologically normal (at least one positive test) and 42 were anergic (no positive test) Among the former, 12 cases (10.8%) showed post-surgical septic complications, while among the latter, fully 24 (57%) suffered from infectious processes. It is concluded that, although numerous factors make patients prone to infection during the postoperative course, immunitary conditions play a very important part. Prophylactic measures are therefore suggested for anergic patient prior to surgery, measures that are essentially represented by parenteral support feeding and immunostimulation.