Surgical antimicrobial prophylaxis was performed with a controlled study on 859 evaluable patients randomized into two groups treated with chemoprophylaxis only or chemo- and immunoprophylaxis in colorectal surgery. Immuno and chemoprophylactic treatment (425 patients) consisted of 70 mg i.m. thymostimulin per day for 7 days beginning 48 h before surgery plus 2 g cefotetan at the moment of induction of anesthesia; the other group (434 patients) received only the single dose of antibiotic. Results in the two different groups were significantly different regarding abdominal abscess and the total infectious episodes in the surgical site with lower frequency in patients receiving both thymostimulin and the antibiotic (cefotetan). Moreover the respiratory tract infections were more than double in those patients not treated with perioperative immunotherapy. Stratifying patients on the basis of grade of skin test reaction, we observed a significantly lower percentage of surgical site infection in hypoergic patients receiving chemo- and immunoprophylaxis.