BACKGROUND New macrolides with improved pharmacokinetic characteristics have been developed and introduced for clinical use in Taiwan. In order to understand the antibacterial activities of these new macrolides, we tested their in vitro activities against common pathogenic bacteria. METHODS Minimum inhibitory concentrations (MICs) of azithromycin, clarithromycin, dirithromycin, erythromycin and roxithromycin for clinical isolates collected from six teaching hospitals in Taiwan were determined by the agar dilution method. The tested bacteria included methicillin-sensitive and -resistant coagulase-negative staphylococci, methicillin-sensitive and -resistant Staphylococcus aureus, viridans streptococci, Streptococcus pneumoniae, Streptococcus pyogenes, Enterococcus spp, Corynebacterium spp, Haemophilus influenzae, Moraxella catarrhalis and Bacteroides fragilis. RESULTS High MICs were detected against most of the bacteria tested except for H influenzae and M catarrhalis. The MIC90 for viridans streptococci, S pneumoniae, S pyogenes, Enterococcus spp, S aureus (both methicillin-sensitive and -resistant), coagulase-negative staphylococci (both methicillin-sensitive and -resistant), Coryne-bacterium spp, and B fragilis were all at least 128 micrograms/ml. Wide ranges of MICs were demonstrated. CONCLUSIONS Most bacteria tested were highly resistant to macrolides. This result is a warning for clinicians that rational use of antibiotics, including macrolides, is mandatory.