BRL 28500 is an injectable antibiotic formulation of ticarcillin (TIPC) as the disodium salt and clavulanic acid (CVA) as the potassium salt in the ratio of 15 parts to 1 part. The formulation with the β-lactamase inhibitor CVA is intended to protect for the TIPC from inactivation by many kinds of β-lactamase.<BR>MICs of BRL 28500 against 252 strains of clinical isolates were compared with those of TIPC, CVA, piperacillin (PIPC), cefoperazone (CPZ) and TIPC+CVA 2μg/ml.<BR>In clinical studies, BRL 28500 was administered to 17 patients with chronic complicated urinary tract infections, and the clinical efficacy, safety and usefulness was assessed.<BR>1) The antibacterial activity of BRL 28500 against three types of Gram-positive coccus (9 strains of S. aureus, 13 of S. epidermidis; 29 of E. faecalis) was approximately equal to that of TIPC and inferior to that of either PIPC or CPZ. Against 29 strains of E. coli, 24 of K. pneumoniae, 7 of P.vulgaris and 8 of P. rettgeri, the antibacterial activity of BRL 28500 was superior to that of TIPC but was approximately equal to that of TIPC against 29 strains of P. mirabilis, 10 of M. morganii, 13 of E. cloacae, 14 of C. freundii, 18 of S. marcescens and 49 of P. aeruginosa.<BR>2) BRL 28500 was administered to 17 patients with chronic complicated urinary tract infection. According to the criteria proposed by the UTI committee of Japan, clinical responses were evaluated as excellent in 3 cases, moderate in 3 and poor in 5 and the overall clinical efficacy rate was thus 54. 5%(6/11). Against monomicrobial infections, the clinical response was 80%(4/5), but against polymicrobial infection it was only 33. 3%(2/6). Regarding the bacteriological response, the eradication rate was 64. 7%(11/17) overall but 5 of 9 strains of P. aeruginosa persisted after treatment.<BR>3) No side effects were observed in any patient and abnormal laboratory examination values were limited to mild increases of Al-P, AI-P and γ-GTP, and GOT in one case.<BR>It was concluded that BRL 28500 was superior to TIPC and was very useful for the treatment of urinary tract infections except in these infections due to P. aeruginosa.