The study relates to the theory of diffusion methods for antibiotic sensitivity testing. The aim of the study was to show the relationship between the antibiotic critical concentration (Cc) and its minimum inhibitory concentration (MIC). The results contribute to the explanation of the Etest's reliability and support the scientific basis for MIC determination using agar diffusion methods. Susceptibility among 90 clinical isolates of 12 common aerobic bacterial species to gentamicin, erythromycin, or oxacillin was assessed using the multidisc method (for Cc), by the agar dilution method (for MIC) and by the Etest. The results of all three methods were statistically compared and found to be closely related. The regression equation for Cc values and MIC was log2(MIC) = 0.99 x log2(Cc)-0.13; r = 0.99; p < 0.05; the regression equation for Cc values and Etest-MIC (Et) was log2(Et) = 0.86 x log2 (Cc)+0.34; r = 0.96; p < 0.05; the regression equation for Etest-MIC values and MIC was log2(MIC) = 1.12 x log2(Et)-0.50; r = 0.96; p < 0.05.