The in-vitro susceptibilities of 13 antimicrobial agents were determined for 207 isolates of viridans group streptococci recovered from patients with significant infections in Taiwan during 1995 and 1997. Variable degrees of susceptibility existed among nine species. High-level penicillin resistance (MIC > or = 4.0 mg/L) was found most frequently in Streptococcus oralis (35%), followed by Streptococcus mitis (20%) and Streptococcus salivarius (8%). However, S. salivarius showed the lowest rate of susceptibility to penicillin (50%). Macrolide resistance also occurred most frequently in S. oralis isolates (55%) but in none of Streptococcus mutans. Penicillin and macrolides tended to be less active against isolates recovered from non-invasive sites than against those isolated from invasive sites. Imipenem was the most active beta-lactam against penicillin-resistant isolates. Ofloxacin, vancomycin and teicoplanin showed good in-vitro activity against all isolates, with MIC90s of 2, 1 and 0.25 mg/L, respectively. None of these isolates displayed high-level resistance to gentamicin and most isolates were susceptible to chloramphenicol. These results indicate the species-related variability of susceptibility, especially to penicillin, macrolides and tetracycline. In addition to S. mitis, S. oralis also displayed high rates of resistance to penicillin and macrolides. The difference in susceptibilities between species of viridans streptococci indicates the importance of accurate identification and the need for continuing surveillance of antimicrobial resistance.