Thienamycin, a natural product produced by Streptomyces cattleya is the first representative of a unique class of β-lactam antibiotics, the carbapenems. Despite its outstanding potency and antibacterial spectrum, thienamycin was itself unsuited for further development because of its chemical instability in concentrated solution and in the solid state. Synthesis of the amidine derivative, N -formimidoyl thienamycin (imipenem, MK0787) resulted in a crystalline product with much improved stability and with antibacterial properties significantly superior to thienamycin. Imipenem has an unusually broad antimicrobial spectrum. A high order of bactericidal activity is found against Pseudomonas aeruginosa, Serratia, Bacteroides fragilis , enterococci and numerous other species intrinsically resistant to other antibiotics. Imipenem is refractory to hydrolysis by all important classes of bacterial β-lactamases and thus exhibits no cross-resistance with penicillins or cephalosporins. Imipenem is distinguished from the new generation of extended-spectrum cephems by its unusually high potency against Gram-positive as well as Gram-negative organisms. Offsetting these excellent antimicrobial properties was an unusual susceptibility exhibited by imipenem to renal metabolism in animal species and in man. Very low urinary recoveries resulted without, however, any significant reduction in the serum half-life of imipenem. A brush-border dipeptidase, dehydropeptidase-I, was shown to be responsible for renal metabolism. Metabolism has been countered with the development of cilastatin (MK0791), a substituted amino-propenoate inhibitor of dehydropeptidase which is specific, potent and well matched in its pharmacokinetic properties for co-administration with imipenem. With the imipenem/cilastatin combination, uniformly high urinary concentrations and recovery are obtained regardless of the varying but often extensive metabolism suffered by imipenem in human populations. An additional benefit conferred by cilastatin results from its ability to exclude imipenem competitively from entry into and subsequent metabolism within the proximal tubular epithelium of the kidney. The tubular necrosis induced by imipenem alone when it is administered at very high doses to susceptible mammalian species is thereby eliminated. Thus the imipenem/cilastatin combination affords reliability and enhanced safety in the application of the antibiotic's unusual antibacterial potential in the treatment of difficult infections regardless of the site of disease.