The ß-lactamase stability and interactions of imipenem were analysed in comparison with those of cefazolin, cefuroxime, cefoxitin, cefotaxime, ceftazidime, mezlocillin, piperacillin and penicillin G for a set of representative ß-lactamases. These enzymes included penicillinases such as those obtained from Staphylococcus aureus, Escherichia coli and other Enterobacteriaceae (TEM-1 and similar enzymes) (group A); cephalosporinases produced by Esch. coli (Amp C type), Serratia liquefaciens, Enterobacter cloacae, Pseudomonas aeruginosa (group B); and ß-lactamases produced by Klebsiella spp., Proteus vulgaris and Bacteroides fragilis and with a high hydrolytic activity for the newer cephalosporins (group C). Enzymes o group A were demonstrated to be highly active against penicillins and also against the early cephalosporins; enzymes of group B showed hydrolytic activity for all other tested compounds, including the newer cephalosporins and cephamycins, but not imipenem, whereas enzymes of group C were highly active against the new cephalosporins but not against cephamycins and imipenem. In conclusion, imipenem shows a moderate affinity for all these enzymes but no detectable hydrolysis.