The interactions of a meropenem were studied with a set of β -lactamases including the new TEM- and SHV-related plasmid-mediated enzymes that have extended-spectrum activity against third-generation cephalosporins (‘cefotaximases’ and ‘ceftazidimases’). Meropenem and imipenem were highly resistant to the hydrolytic activity of all the TEM and SH V related β -lactamases, and to the OXA enzymes, as were the cephamycins: cefoxitin and cefotetan. The two carbapenems were also highly stable to Class C β -lactamases (chromosomal cephalosporinases) whereas third-generation cephalosporins and cephamycins were slowly hydrolyzed. Both carbapenems demonstrated quite similar affinities for all the enzymes studied. In some instances, and particularly with Class A (TEM- and SHV-derived) enzymes, meropenem inactivated the β -lactamase activity. Imipenem appeared less reactive in this respect.