We evaluated the efficacy of a new quinolone, DU-6859a, using a new model of rat uterine endometritis. Rats were infected with mixed inocula of Escherichia coli and Bacteroides fragilis. The MICs of DU-6859a and levofloxacin against E. coli were 0.025 and 0.05 mg/L, respectively; those against B. fragilis were 0.20 and 0.39 mg/L, respectively. Immediately after inoculating 10(5) cfu/rat of each organism, DU-6859a or levofloxacin (20 mg/kg po bid or tid, respectively, for 3 days) was administered and compared with the untreated group. The viable cell counts of E. coli and B. fragilis in the DU-6859a- and levofloxacin-treated groups were significantly lower than those in the untreated group. These results suggest that DU-6859a would be useful for treating polymicrobial infections in uterine endometritis.