One objection occasionally raised against the use of routine antibiotic prophylaxis for pelvic surgery is the concern that the flora of treated patients will be altered, thus favoring the emergence of different, potentially more resistant organisms. This report summarizes experience with 100 women undergoing elective hysterectomy who were subjected to short-term cefazolin, penicillin, or placebo prophylaxis. The changes in preoperative versus postoperative flora were similar among antibiotic-treated and placebo-treated groups. In general, postoperative isolation of gram-negative aerobes and Bacteroides sp increased, and isolation of gram-positive aerobes (except group D streptococci) decreased in antibiotic-treated and placebo-treated groups. The clinical implications of these findings arc discussed.