The induction potency of various beta-lactams as well as that of 'nonspecific' inducers such as the media employed or body fluids were studied in gram-negative clinical isolates and in their resistant corresponding counterparts. In all wild-type isolates quite a few beta-lactams (mainly cefoxitin and imipenem) shared the ability to induce the chromosomal beta-lactamase, whereas all beta-lactams - including 6-APS and 7-CPS clavulanic acid, and sulbactam (both beta-lactamase inhibitors), exhibited strong induction potency in their corresponding resistant counterparts. Moreover, all resistant counterparts exhibited the phenomenon of 'nonspecific' induction, whereas the wild-type strains did not. Interestingly, in both Proteus vulgaris 4917 W and Providencia rettgeri 862 W wild-type strains most beta-lactam compounds were potent inducers, thus providing an explanation for resistance against beta-lactamase-unstable compounds. The addition of subinhibitory concentrations of the quinolone compound enoxacin or chloramphenicol did not influence the phenotypic beta-lactamase expression, neither in wild-type strains nor in their resistant counterparts, whereas the addition of clindamycin or gentamicin resulted in a marked decrease of enzyme production in cephalosporinase overproducers, the P. vulgaris and the P. rettgeri isolates.