The effect of ciprofloxacin and two marRAB inducers on the susceptibility to β-lactam antibiotics in two AmpC β-lactamase semiconstitutive producer Citrobacter freundii clinical isolates (the DM1 and DM2 strains) was studied. Possible changes in outer membrane protein expression, permeability to cephaloridine, active efflux, and hydrolytic activity of β-lactamase-crude extracts were evaluated under the influence of ciprofloxacin, sodium salicylate, and 2,4-dinitrophenol. Results were compared with those of the effect of the same three chemicals on a normally β-lactamase-inducible wild-type C. freundii strain. The three assayed compounds decreased β-lactamase hydrolysis on cephaloridine in both the two clinical isolates as well as in the wild-type strain. However, only the DM1 and DM2 strains showed increased susceptibility to β-lactams. Sodium salicylate and 2,4-dinitrophenol, but not ciprofloxacin, reduced the expression of a 45-kDa outer membrane protein in the three studied strains, which was accompanied by a 4- to 20-fold diminution in permeability to cephaloridine. In conclusion, two marRAB inducers and ciprofloxacin induced the Mar phenotype and repressed AmpC β-lactamase synthesis in the DM1 and DM2 clinical isolates. © Mary Ann Liebert, Inc.