Evaluation of antibacterial activity, a pharmacokinetic study and a clinical study were performed to evaluate imipenem/cilastatin sodium (MK-0787/MK-0791), a combination of a new β-lactam antibiotic (MK-0787) and a renal dipeptidase inhibitor (MK-0791).<BR>MICs of MK-0787, CPZ, LMOX, CAZ and GM (or CFS, ABPC) against 432 clinical isolates, which are stored in our laboratory, were measured by the Standard Method of the Japan Society of Chemotherapy and the results were compared. CAZ, CFS and MK-0787 had the best activity against P. aeruginosa, with little difference among the three agents. CAZ had the best activity against S. marcescens and MK-0787 followed. MK-0787 had the best activity against E. cloacae, C. freundii, S. aureus, S. e pidermidis and E. faecalis. MK-0787 had similar activity to LMOX and CAZ against E. coil and K. pneumoniae. MK-0787 had the best activity against Proteus sp. next to LM OX and CAZ and showed broad and potent antibacterial activity.<BR>500 mg of MK-0787 and 500 mg/500 mg of MK-0787/MK-0791 were administered by a 30-minute drip infusion to 5 healthy volunteers using a cross-over fashion. Peak levels in plasma of MK-0787 and MK-0787/MK-0791 were 35.2 and 36.0/35.1μg/ml within 6.5 hours, and the t_1/2 was 0.8 and 0.9/0.8 hours, respectively. Urinary recovery of MK-0787 was 16.0% after MK-0787 alone and 64.6% after the combination. The latter value demonstrated the effect of MK-0791.<BR>MK-0787/MK-0791 was administered to 24 patients with chronic complicated urinary tract infections and 3 with prostatitis. In 20 patients who satisfied UTI Drug Evaluation Star. dards, 9 were evaluated as excellent, 9 as good and 2 as no response with an efficacy rate of 90%. No adverse effects or abnormalities of laboratory findings were observed in these 27 patients.<BR>From these results, it was concluded that MK-0787/MK-0791 is a useful agent for the treatment of urinary tract infections.