The in-vitro activity of HMR 3647, a novel ketolide, was investigated in comparison with those of erythromycin A, roxithromycin, clarithromycin (14-membered ring macrolides), amoxycillin-clavulanate and ciprofloxacin against 719 recent clinical Gram-positive, Gram-negative and anaerobic isolates and type cultures. HMR 3647 generally demonstrated greater activity than the other compounds with MIC90s of < or =0.5 mg/L, except for Staphylococcus epidermidis (MIC90 > 128 mg/L), Haemophilus influenzae (MIC90 = 2 mg/L), Enterococcus faecalis (MIC90 = 2 mg/L), Enterococcus faecium (MIC90 = 1 mg/L) and the anaerobes, Bacteroides fragilis (MIC90 = 2 mg/L) and Clostridium difficile (MIC90 = 1 mg/L). In general, an increase in the size of the inoculum from 10(4) to 10(6) cfu on selected strains had little effect on the MICs of HMR 3647. Additionally, the in-vitro activity of HMR 3647 was not affected by the presence of either 20 or 70% (v/v) human serum. The antichlamydial activity of HMR 3647 was generally greater than that of commonly used antichlamydial antimicrobials.