To determine the susceptibility to macrolides of common pathogenic bacteria isolated from Taiwan, the in vitro activities of erythromycin, roxithromycin, azithromycin, clarithromycin, and dirithromycin were tested against 492 clinical isolates of eight different bacteria, collected from the National Taiwan University Hospital. The results showed high minimum inhibitory concentrations (MICs) against most of the tested bacteria. The MIC90s for Staphylococcus aureus (both methicillin-resistant and -sensitive strains), coagulase-negative staphylococci (both methicillin-resistant and -sensitive strains), Streptococcus pyogenes, Streptococcus pneumoniae, enterococci, peptostreptococci, and Bacteroides fragilis were all > or = 256 micrograms/ml. The MIC50s for methicillin-resistant strains of S. aureus and coagulase-negative staphylococci, and enterococci were > or = 256 micrograms/ml. For S. pneumoniae, peptostreptococci, and B. fragilis, the MIC50s were > 8 micrograms/ml. The resistance rates to macrolides were 80% or more in methicillin-resistant staphylococci and about 30% in methicillin-sensitive staphylococci. Around 55% of S. pneumoniae strains and 37 approximately 42% of S. pyogenes strains were resistant to macrolides. Cross-resistance to different macrolides was clearly demonstrated in most of the resistant strains.