The in vitro activity of PD 127,391, a dihalogenated quinolone, was compared with those of ofloxacin, ciprofloxacin, nalidixic acid, gentamicin, and cefuroxime against 525 recent isolates and well-characterized antimicrobial agent-resistant strains. The MICs of PD 127,391 against 90% of members of the family Enterobacteriaceae, Bacteroides fragilis, Haemophilus influenzae, Neisseria sp., and Streptococcus pneumoniae were less than or equal to 0.12 microgram/ml. Some 90% of Pseudomonas aeruginosa and staphylococci were susceptible to 0.25 micrograms of PD 127,391 per ml. Against most strains, PD 127,391 was 2- to 8-fold more active than ciprofloxacin, but it was 64-fold more active than ciprofloxacin against B. fragilis. Strains of members of the family Enterobacteriaceae which were resistant to nalidixic acid were less susceptible to all of the quinolones tested, including PD 127,391. The MIC and minimum lethal concentration of PD 127,391 against three strains of Chlamydia trachomatis were each 0.06 microgram/ml, and the MIC against 90% of 21 strains of Mycobacterium tuberculosis was 1 microgram/ml. PD 127,391 was less active at pH 5, its maximal activity being at pH 7 to 8. The presence of urine at pH 5.9 decreased the bactericidal activity. The protein binding of PD 127,391 was 2 to 7%, and serum had little effect on activity.