The in-vitro activity of CG 5501 against a wide range of recent clinical isolates was compared with that of three fluoroquinolones. CG 5501 inhibited 90% of the species of the family Enterobacteriaceae at 0.5 mg/L or less, exceptions being Enterobacter spp. (MIC90 2 mg/L) and Serratia spp. (MIC90 4 mg/L). Ninety per cent of Pseudomonas aeruginosa, Stenotrophomonas maltophilia and Acinetobacter spp. were inhibited by 16, 4 and 1 mg/L respectively. CG 5501 had high activity against Gram-positive cocci, 90% of staphylococci being inhibited at 2 mg/L. Methicillin-resistant Staphylococcus aureus strains were generally ciprofloxacin-resistant yet were all susceptible to 4 mg/L or less of CG 5501. Isolates of Streptococcus pneumoniae were eight-fold more susceptible to CG 5501 (MIC90 0.5 mg/L) than to ciprofloxacin (MIC90 4 mg/L) and the former had a similar activity to that of trovafloxacin and sparfloxacin. Enterococcus faecalis was generally two- to four-fold more susceptible to CG 5501 or trovafloxacin than to ciprofloxacin. CG 5501 and trovafloxacin had high activity against Bacteroides fragilis (MIC90 0.25 mg/L). Five strains of Chlamydia spp. were inhibited by < or =0.12 mg/L of CG 5501; sensitive and multiresistant strains of Mycobacterium tuberculosis were inhibited by < or =0.5 mg/L of CG 5501. The high activity and breadth of its antibacterial spectrum suggests that CG 5501 should be useful in a wide range of clinical infections.