Over a one-year period (November 1996 to November 1997), in the Emergency Surgery Clinic perioperative parenteral antibiotic prophylaxis during colorectal operations is performed in a series of 32 patients, admitted on an emergency and deferred urgency basis, or for routine operative treatment. Of them 30 present malignant processes involving colon and rectum, and two--inflammatory diseases. All patients receive single i.v. injections with Cephalothin/Cefazolin at dose 2.0 g and Metronidazole 0.5 g immediately after anesthesia induction. In nine cases additional early treatment is necessitated--within 24 hours after the operative intervention--by administration of the same drug combination 4 times at 6-hour intervals, and in another two prophylaxis is substituted for continuous 5-day treatment using the same therapeutic scheme. In 30 patients the postoperative period runs a course free of noteworthy complications. In two instances there is evidence of operative wound suppuration, and in further two--urinary tract infection development unrelated to the antibiotic prophylaxis applied. The specific features characterizing the application of antibiotic prophylaxis during colorectal surgery are discussed, and appropriate drug therapy schemes are recommended, consistent with worldwide and Bulgarian experience along this line, as well as with the concrete hospital and economical conditions in this country. Special emphasis is laid on two aspects: optimization of the timing of antibiotic agent/agents injection, and reaching peak serum and tissue bactericidal concentrations in the immediate vicinity of the surgical incision; optimization of the duration of antibiotic prophylaxis on the ground of well established indications for the application of antibiotic prophylaxis in colorectal surgery in compliance with the dynamic patterns of intra- and postoperative septic risk. The modest number of patients subjected to updated parenteral perioperative antibiotic prophylaxis does not warrant a definitive interpretation of the data from the comparative clinical and pharmaco-economical analyses performed. Nevertheless, the preliminary results point to the economical expedience of the therapeutic approach suggested.