Intracellular beta-lactamase activity of gram-negative rods (four Pseudomonas aeruginosa strains, one strain each of Enterobacter cloacae, Proteus rettgeri, Providencia stuartii and Serratia marcescens) was greatly increased by subinhibitory concentrations of cefoxitin, with the exception of one P. aeruginosa strain. Cefotaxime, cefoperazone and ceftazidime were much less effective as enzyme inducers in these strains. A reduction in beta-lactamase activity after pre-incubation with cephalosporins was observed for particular strains and substances. Susceptibility to beta-lactamases decreased in the following sequence: cephalothin, cefoperazone, cefotaxime, ceftazidime, cefoxitin. There was a good correlation between the degradation of cephalosporins in uninduced cells of Enterobacteriaceae and the minimal inhibitory concentrations except for cefoxitin.