Comparative study of twenty two strains of Escherichia coli producing a well-characterized penicillinase of the OXA type (oxacillinase) or of the IRT type (TEM resistant to inhibitors) evidenced different criteria leading to the distinction of two groups of strains, according to the type of the harboured enzyme. Applied to eleven clinical strains, these criteria were always concordant with the determination of the isoelectric point, of hybridization and of oligotyping using probes for the classification into OXA or IRT type. We therefore propose the measurement of three inhibition diameters on antibiogram (cefepime, mecillinam and ceftazidime) for the routine distinction of the two enzymes. The interest of the easy characterization of these enzymes is reinforced by our findings that, as for all OXA strains, the inoculum effect which is displayed by some third-generation cephalosporins is very important and should be taken into account in the treatment of severe infections.