In a prospective, double-blind, randomized study of patients subjected to gastric surgery, parenteral cefazolin begun before operation significantly reduced the would infection rate. Infection developed only when incisional contamination occurred during operation and either when the antibiotic was not present in the tissues of the wound at the time of inoculation or when the offending bacteria were already resistant to the antimicrobial used. To be reliably effective, antimicrobial prophylaxis should always be instituted preoperatively in patients at high risk of infection, ie, those with conditions not associated with gastric hyperacidity. On the other hand, since patients with duodenal ulcer disease and resulting gastric hypersecretion usually have a relatively sterile stomach lumen, antibiotic prophylaxis for gastric operations in these cases is probably not indicated.