BAY 12-8039 is a new 8-methoxyquinolone with antibacterial activity against gram-positive bacteria which is significantly better than those of sparfloxacin or ciprofloxacin. The minimal inhibitory concentrations (MICs) for 90% of methicillin-susceptible Staphylococcus aureus and Staphylococcus epidermidis were 0.062 and 2 mg/l, respectively. The MIC90s for ciprofloxacin-resistant, methicillin-susceptible and methicillin-resistant S. aureus were 8 mg/l. Against the staphylococcal strains tested sparfloxacin was 2-fold and ciprofloxacin > or = 10-fold less active. MIC90s for Streptococcus pneumoniae, Streptococcus pyogenes and Streptococcus agalactiae were 0.125-0.5 mg/l, irrespective of whether strains with diminished ciprofloxacin susceptibility or ciprofloxacin-susceptible strains were tested. Against the streptococci sparfloxacin was 2- to 4-fold less active. Against gram-negative bacteria BAY 12-8039 is almost as active as ciprofloxacin, except for Pseudomonas aeruginosa. Against Bacteroides fragilis, Bacteroides spp. and Clostridium spp. BAY 12-8039 was as active as metronidazole. The bactericidal activity against S. aureus and S. pneumoniae was in contrast to that of the other quinolones tested, penicillin G, amoxicillin+/-clavulanate, cefuroxime and clarithromycin, concentration-dependent. As compared to ciprofloxacin, development of resistance was less pronounced. The spontaneous mutation frequency towards BAY 12-8039 resistance was 2.8 x 10(-8) in Escherichia coli, 7.06 x 10(-8) in S. aureus and < 1.4 x 10(-9) in S. pneumoniae.