beta-Lactamases, major determinants of bacterial resistance to beta-lactam antibiotics, can be classified into specific molecular classes following identification of active-site amino acid or nucleotide sequences. The use of gene probes for epidemiologic purposes is becoming commoner. A semiempirical classification scheme has been proposed using substrate profiles and inhibition by clavulanic acid and aztreonam as criteria. Class 1 cephalosporinases are potently inhibited by aztreonam but poorly inhibited by clavulanate, whereas class 2 penicillinases and broad-spectrum beta-lactamases have very poor affinities for aztreonam but are inhibited by clavulanic acid. Class 3 beta-lactamases include the metalloenzymes. Resistance to beta-lactam antibiotics can be related to many beta-lactamase-mediated phenomena, including increased frequency of beta-lactamase production in clinical isolates, wider distribution of beta-lactamase-mediating plasmids, production of multiple beta-lactamases, induction of chromosomal class 1 cephalosporinases, selection of depressed mutants for production of class 1 enzymes, leakage of beta-lactamase from gram-negative organisms, functions of penicillin-binding proteins as beta-lactamases, and identification of novel beta-lactamases.