Minimum inhibitory concentrations (MIC) of new quinolones against both Gram-positive and Gram-negative bacteria obtained using agar-dilution and broth-microdilution procedures were compared. A primary regression curve and a correlation coefficient (r) between 2 MICs for each of ofloxacin, ciprofloxacin, tosufloxacin, sparfloxacin or balofloxacin were calculated. For the 5 quinolones, average correlation coefficients were 0.891 for the Gram-positive bacteria tested, and 0.865 for the Gram-negative bacteria. The range of, correlation coefficients for the Gram-positive bacteria for these drugs was from 0.835 to 0.919, and that for the Gram-negative bacteria was from 0.815 to 0.865. From these data, it is clear that there is a good correlation between the 2 MICs of the new quinolones obtained using the agar-dilution and the broth-microdilution procedures. It was also shown that the value of the slope of the regression curves were nearly the same for the 5 quinolones tested. However, some particular strains of Morganella morganii, Pseudomonas cepacia, Xanthomonas maltophilia, Pseudomonas aeruginosa and Streptococcus pneumoniae exhibited different correlation coefficients from other strains.