The in-vitro activity of two new quinolone antimicrobials, rufloxacin and MF 961, together with the desmethylated metabolite of rufloxacin (MF 922) were compared with other orally administered agents against 622 bacterial strains. Against Enterobacteriaceae and Pseudomonas aeruginosa rufloxacin was generally active (MIC90 1-8 mg/L) with the exception of Klebsiella and Serratia spp. (MIC90 32 mg/L and Enterobacter spp. (MIC90) 64 mg/L. The respiratory pathogens Haemophilus influenzae and Moraxella catarrhalis were susceptible to rufloxacin (MIC90 0.5 and 1 mg/L respectively) but Streptococcus pneumoniae was less susceptible (MIC90 32 mg/L). Staphylococcus aureus were susceptible to rufloxacin (MIC90 2 mg/L). The rufloxacin metabolite MF 922 was generally as active as its parent. MF 961 was usually two-fold more active than rufloxacin. All three compounds were four to 16 times less active than norfloxacin, but rufloxacin was as active or somewhat more active than norfloxacin against Staphylococcus spp. Any strains showing decreased susceptibility to other quinolones exhibited cross resistance to these new agents. The MBC of rufloxacin and MF 922 was within one dilution of the MIC and human serum had little effect upon the activity of both agents. The protein binding of rufloxacin and MF 922 at 1 and 10 mg/L were 55% and 63.8% and 30.3% and 32.6% respectively. The activity of rufloxacin against four strains of Chlamydia trachomatis and one strain of Chlamydia pneumoniae was determined. The MICs for C. trachomatis were 4-8 mg/L and 4 mg/L for C. pneumoniae.