The inhibitory and bactericidal activities of meropenem and imipenem were compared using Acinetobacter spp. and Enterobacteriaceae resistant to second generation cephalosporins. The inhibitory activity of both agents was similar for Acinetobacter spp. but meropenem was more active against the Enterobacteriaceae. Bactericidal activities were compared employing time-kill curves which indicated meropenem was more bactericidal than imipenem against Enterobacteriaceae at pharmacologically achievable concentrations while imipenem was more bactericidal than meropenem against Acinetobacter spp.