There are no current data regarding the effect of a newer, broad-spectrum penicillin on lower reproductive flora at hysterectomy. To identify any existing differential effect on species and their susceptibilities, we obtained pre- and postoperative lower reproductive tract culture material from 209 women who were given single-dose piperacillin, then placebo or triple-dose cefoxitin, intravenously for prophylaxis at vaginal and abdominal hysterectomy in a prospective, randomized, blinded clinical trial. Significantly more preoperative endocervical bacteria were susceptible to piperacillin. Piperacillin caused less alteration in the numbers of lower reproductive tract flora when preoperative species were compared with postoperative species. More resistance to cefoxitin was identified postoperatively in bacteria recovered from the vaginal cuff of women who remained uninfected.