The difficult problem of the choice of antibiotic, when treatment is assumed to be necessary for prevention of postoperative infection, is discussed in relation to published data and the rules for prescribing antibiotics. Complications of antibiotic therapy are summarized, and the principal etiologic bacteria, as a function of the type of operation, are described. Surgical acts can be grouped in four categories as a function of the frequency with which they are followed by infection. Criteria of choice of antibiotic and the advantages and inconveniences of preventive antibiotic treatment are outlined. Results of the main controlled studies are reviewed in relation to the type of surgery performed, and an attempt made to choose from among the most effective products. As the ecological risk appears to be of primary importance, narrow spectrum antibiotics are preferred, as they have the minimum effect on host flora. These include penicillin G for certain gynecological operations, as well as in ENT surgical procedures, and metronidazole (Flagyl) for lower digestive tract surgery or gynecological procedures.