The individual results of the antibiotic susceptibility tests of 110 Enterobacteriaceae to 31 antibiotics (beta-lactamines, aminoglycosides, quinolones) have been compared on Mueller-Hinton and Isosensitest. No difference between the diameters of inhibition has been shown by the paired t test or signed rank test. The identification of the phenotypes of resistance (resistance patterns) were compared by principal component analysis and by help of an expert system. The patterns are identical in 99 cases and different in 11 cases. Six pattern were erroneous on Mueller-Hinton and 5 on Isosensitest. Globally both media gave a similar comparable answer in these conditions.