Resistance of Enterobacter, Serratia and pseudomonas strains to newer cephalosporins is often associated with stable derepression of synthesis of the chromosomal betalactamases. Similar resistance is developed by enzyme inducible strains in response to betalactamases inducers. This finding poses many clinical problems including emergence of resistance during therapy with the drugs. In this study we evaluated the MICs of several new betalactam compounds against 76 Enterobacter, Serratia and Pseudomonas strains before and after cefoxitin-induction of betalactamases. The MICs against several Enterobacter strains (45%) after cefoxitin induction were elevated four fold or more. Serratia strains showed no significant variations of the MICs after cefoxitin induction. The MICs of piperacillin against many Pseudomonas strains (78%) after cefoxitin induction were elevated four fold or more. These data were confirmed using cefoxitin disk approximation test. Outbreaks of nosocominal infection with these multiresistant bacteria and spread of the strains throughout the hospital are already being seen. Control of these problems can only be achieved through the judicious and restricted use of these new antibiotics.