The resistance of bacteria, particularly Gram-negative bacteria, to beta-lactam antibiotics is principally caused by enzymes. Beta-lactamases inactivate these antibiotics by opening, more or less rapidly, the beta-lactam ring. The chronology of therapeutic discoveries is governed by natural and acquired resistance. The first step was to establish the characteristics of beta-lactamases (location, biogenesis, enzymatic profile, affinity constant, inhibition profile, isoelectric point, molecular weight, genetic determination, etc.). Advances in the selection of natural or semi-synthetic compound are centred on the following points: increased stability to beta-lactamases, inhibitory effect, reduced inducibility, low affinity for the enzyme, greater speed of penetration through the bacterial wall, increased tropism for targets. Notable among the new beta-lactam antibiotics are acylureidopenicillins, beta-lactamase inhibitors and carbapenems. The antibacterial activity of third generation cephalosporins is 10 to 1000 times higher than that of previous cephalosporins; the result is a wider spectrum including, in particular, cephalosporinase-producing organisms. Concerning acquired resistance, the behaviour of new antibiotics must be examined by comparing the minimum inhibitory concentrations of isogeneic and clinical strains, and according to phenotype or mechanism: sensitive, penicillinase producer, cephalosporinase producer or producer of both penicillinases and cephalosporinases. During synthesis of a penicillinase, penicillins and some cephalosporins are more or less inactivated. With a constitutive cephalosporinase, all cephalosporins are inactivated as are, to a lesser degree, penicillins and monobactams, e.g. aztreonam. The emergence of new enzymes, including broad-spectrum beta-lactamases, and their extension to sensitive bacteria show that the enzymatic mechanism still has potentials for development.