The emergence of resistance to multiple beta-lactam antibiotics is a major problem in patients infected with organisms that characteristically produce inducible beta-lactamases--e.g., species of Pseudomonas, Enterobacter, Serratia, Citrobacter, indole-positive Proteus, and Providencia. Resistance has emerged in 14%-56% of patients infected with these organisms and treated with one of the newer cephalosporins. The emergence of resistance has been associated with clinical failure or relapse in 25%-75% of these patients. Combination therapy appears to have little impact on rates of resistance or its clinical consequences. Multiply resistant organisms have spread widely in some hospitals, and this trend has been correlated closely with the extent of use of the newer cephalosporins. The magnitude of the problem is frequently underestimated because many properly performed susceptibility tests fail to detect resistance when it is actually present and because some methods used to calculate rates minimize the impact of the emergence of resistance among organisms with inducible beta-lactamases.