Xanthomonas maltophilia produces two inducible beta-lactamases, L1 and L2, and resists the antimicrobial activity of beta-lactam antibiotics, including carbapenems. L1 is a zinc-metaloenzyme with carbapenemase activity; L2 is an unusual cephalosporinase. Mutant strains with high- and low-level constitutive expression of these enzymes were derived from three reference strains of X. maltophilia. With a single exception, the mutant strains had altered expression of both enzymes, indicating that these beta-lactamases share regulatory components. The exception was a mutant strain that had low-level constitutive (basal) expression of L1 enzyme but remained inducible for L2. A parent strain with low-level beta-lactamase inducibility was more susceptible to penicillins, cephalosporins and carbapenems than were those in which higher levels of enzyme activity were inducible. Mutations that caused high-level constitutive beta-lactamase expression increased resistance to penicillins and newer cephalosporins. beta-Lactamase basal mutant strains, including the one that remained inducible for L2 enzyme, were more susceptible than inducible strains to these drugs. Organisms with inducible or high-level constitutive beta-lactamase expression were equally resistant to meropenem and imipenem but basal mutant strains, including the one that remained inducible for L2 enzyme, were more susceptible to meropenem than imipenem. Minimal inhibitory concentrations of meropenem, penicillins and cephalosporins, but not imipenem, were greater on Mueller Hinton agar than on IsoSensitest or Diagnostic Sensitivity Test agars. This behaviour was independent of beta-lactamase inducibility, and may reflect permeability differences between cells grown on different media.