A single dose of clindamycin, cefoxitin, or metronidazole was administered to each of 30 women. The mean concentration of cefoxitin in pelvic fluid at 1 h exceeded those of the other two drugs (P less than 0.007). Cefoxitin concentrations were inferior to those of the other drugs when compared with the published MIC for 90% of Bacteroides fragilis strains.