Of three patients with Enterobacter mediastinitis treated with cefamandole, two failed initial treatment due to emergence of strains resistant to cefamandole and third generation cephalosporins. Extracts of the sensitive strains showed inducible hydrolysis of cephalothin; resistant strains (wild and laboratory derived) showed constitutive hydrolysis of cephalothin and cefamandole and bound but did not hydrolyze cefotaxime and latamoxef (moxalactam). In one mutant, increased latamoxef resistance appeared due to non-enzymatic mechanism(s). The Enterobacter strain from the patient who was treated successfully with cefamandole yielded only low level resistant mutants with lower cephalosporin binding than obtained with the other strains.