Imipenem (N-formimidoyl thienamycin) is the first representative of a new class of β-lactam antibiotics — the carbapenems. Imipenem has an unusually broad spectrum, high potency, and no cross-resistance with other β-lactam antibiotics. Susceptible gramnegative species include Pseudomonas aeruginosa, Serratia, and Enterobacter. Activity is high against Staphylococcus aureus, most group D streptococci, and Staphylococcus epidermidis but is variable against methicillin-resistant S. aureus. Imipenem is more active against Bacteroides than are other β-lactam agents, chloramphenicol, metronidazole, and clindamycin. The minimal inhibitory concentrations (MICs) for 98% of 30,655 isolates — excluding those of the three resistant species (Pseudomonas maltophilia, Pseudomonas cepacia, and Streptococcus faecium) — were P. aeruginosa, MBCs of imipenem are less influenced by high inoculum density than are MBCs of antipseudomonal penicillins and cephalosporins. Stability of imipenem to diverse classes of plasmid-mediated and chromosomal β-lactamases accounts for its lack of cross-resistance with other β-lactam antibiotics. Imipenem is also active against P. aeruginosa with non-lactamase-mediated resistance to classical β-lactam agents. Efficacy of imipenem was shown in animal models, including septicemia in normal and neutropenic rodents and P. aeruginosa pneumonia. Imipenem also has a unique postantibiotic effect against P. aeruginosa in vivo.