A total of 20 patients involved in gastrointestinal surgery was treated with cefoxitin (CFX). Ten patients were treated for postoperative infections and the other 10 patients were given CFX to prevent postoperative infections. The following results were obtained: In 10 patients treated for postoperative infections, 5 responses were judged "excellent", 1 "good", 2 "fair", 1 "poor" and 1 "unknown". In 25 strains of bacteria isolated from these patients, 21 were eradicated, 3 were replaced and 1 was unknown. In 10 patients given CFX for prevention of postoperative infections, 9 were judged "excellent" and the remaining 1 "good". No side effects were observed in any of the patients treated with CFX.