The in vitro spectrum of moxifloxacin (a C-8-methoxyquinolone) was compared to that of ciprofloxacin and levofloxacin against 924 recent clinical isolates from cancer patients. Moxifloxacin was more active than the comparator agents against Gram-positive pathogens, with potent activity against Aerococcus spp., Listeria monocytogenes, Micrococcus spp., Rhodococcus equi, and Stomatococcus mucilaginous, methicillin-susceptible Staphylococcus spp., all beta hemolytic streptococci, viridans streptococci and Streptococcus pneumoniae. It also had good to moderate activity against Bacillus spp., Corynebacterium spp., Enterococcus faecalis, and methicillin-resistant staphylococci. Although ciprofloxacin was the most active agent tested against the Enterobacteriaceae, moxifloxacin inhibited the majority of these isolates at < or =2.0 microg/ml. Moxifloxacin was the least active of the three agents tested against Pseudomonas aeruginosa, but had significant activity against other non-fermentative Gram-negative bacilli including Acinetobacter spp., Flavobacterium spp., Pseudomonas spp. other than P. aeruginosa, and Stenotrophomonas maltophilia. The overall broad spectrum of moxifloxacin, and its availability for both oral and parenteral administration, warrants its evaluation for the prevention and treatment of infections in cancer patients.