42% of 284 ampicllin-resistant enterobacteriaceae revealed no spontaneous production of β-lactamases. 27 strains exhibited the production of more than one single enzyme (mostlyKlebsiella andEnterobacter spp. isolates). The production of identical enzymes does not lead to a uniform resistance pattern, but β-lactamase producing strains showed a higher degree of resistance, especially against the “broad spectrum” penicillins. However, with respect to the newer cephalosporins, induction of chromosomally mediated enzymes largely contributes to the impaired antimicrobial susceptibility; it is above all the amount of enzyme that deserves attention (4, 5).