Bacteria are usually classified as susceptible (S), intermediate (l) or resistant (R) by comparison of their MICs with two reference MIC cutoff levels. Three sets Sc, Ic and Rc are thus constructed. By using agar diffusion, the same bacteria are classified as Sd, Id or Rd, by comparison of inhibition zone diameters with two reference diameter breakpoints. These Sd, Id and Rd sets are not absolutely identical with Sc, Ic and Rc respectively. Diameter breakpoints were submitted to successive adjustments until minimal differences were achieved, i.e. when the greatest possible number of elements was included in the sets: (Formula: see text) Comparison of this method with more conventional techniques emphasizes its value.