The determination of quantitative data (minimal inhibitory concentrations [MIC values]) using the broth microdilution method is considered the method of choice for in-vitro susceptibility testing of bacterial pathogens. For the inoculation of microtiter plates used in the broth microdilution method, an inoculation density of 5x10(5) CFU/ml has to be achieved according to the CLSI standard. In this study, it was shown that the requested CFU/ml corresponded to an optical density of 0.00027 of a bacterial suspension determined in a spectrophotometer at 625 nm (OD625). The comparison of susceptibility data of clinical isolates determined by agar disk diffusion and, in parallel, by broth microdilution confirmed the comparability of both methods. For most antibiotics the frequency of "very major errors" was below 10%. However, a higher frequency of errors was observed for combinations of antimicrobial agents and bacteria for which no valid breakpoints are available. The comparative testing of cephalothin showed a frequency of"very major errors" of 20.3% for staphylococci and 18.4% for Enterobacteriaceae; for apramycin the frequency of "major errors"was found to be 46.0% for Enterobacteriaceae. The reference strains as recommended in the CLSI Standard M31-S1 should be used for quality assurance.