The susceptibilities of 154 clinical isolates of fastidious and unusual bacteria were determined by the Etest, and were compared with the MICs for these strains obtained by the agar dilution method. Antimicrobial agents; benzylpenicillin, ampicillin, cefotaxime, imipenem, erythromycin, clindamycin and minocycline were used in this study. In general, the Etest MICs were rapidly and easily interpreted. The Etest MICs agreed within +/- 1 log2 and +/- 2 log2 dilution, 87.6% and 98.2% respectively in all strains tested. MICs of vancomycin with the Etest for S. pneumoniae and other Streptococci tended to be 1 to 2 log2 dilution higher than those of the agar dilution method. MICs of erythromycin for the other Streptococci also tended to be 1 to 2 log2 dilution higher. However, there were no significant differences for H. influenzae, Campylobacter spp., H. pylori, Capnocytophaga spp., E. corrodens, M. catarrhalis, N. gonorrhoeae, Corynebacterium spp., L. monocytogenes and Anaerobic bacteria. As mentioned above, the Etest represent a potential method for antimicrobial susceptibility testing of these fastidious and unusual bacteria.