A study of 283 isolates of gram-negative bacilli revealed a good correlation ( r = −0.74) between disk diffusion zones of inhibition and agar dilution minimal inhibitory concentrations. Regression analysis suggested that strains with zone sizes 11 mm should be considered resistant, but 34 of 45 strains resistant by minimal inhibitory concentration (including 27 strains of Pseudomonas aeruginosa ) would have been called susceptible using this break point.