As a basic study on 6315-S (flomoxef), a newly developed antibiotic, bacteriological evaluation, absorption and excretion tests were performed in 4 healthy volunteers and in 8 patients with chronic complicated urinary tract infection to clarify its efficacy and safety.<BR>Its antibacterial activity was compared with that of, latamoxef (LMOX), cefotiam (CTM) and cefamandole (CMD) by measuring MIC's. 6315-S proved excellent againstE. coil, K. pneumoniae, P. mirabilisandS. marcescens. Compared with LMOX, it was equally effective againstS. marcescens, superior against indole-positiveProteus, and about three times as effective as LMOX againstS. aureus.<BR>In an absorption and excretion study, 6315-S in a dose of 0.5 g was administered to four healthy volunteers by i. v. drip infusion for 30 min and for 1 h by the cross-over method. Peak serum concentrations were 28.8 and 19.6μg/ml with half-lives of 0.73 and 1.26 h, respectively. Urinary excretion ratios during 8 h were 82.5 and 84.8%, respectively.<BR>Clinical efficacy was evaluated by treating 6 patients with chronic complicated urinary tract infection, 1 patient with acute epididymitis and 1 with prostatitis. Administration was 1 g by i. v. drip infusion, twice daily for 5-6 days. Efficacy against chronic complicated urinary tract infections (UTI) was evaluated in 6 patients according to the “Criteria of the UTI Committee”.<BR>The results were: excellent in 2, good in 1 and poor in 3, with an overall efficacy rate of 50%(In 2 other patients, as diagnosed by the attendant physicians, results were good 1, and poor 1).<BR>Bacteriologically, 10 strains were isolated from the above 6 patients of which 8 strains were eradicated, but 2 strains ofP. aeruginosaremained and 2 other strains ofP. aeruginosareplaced other bacteria.<BR>Although no specific side-effects were recognized, a rise in GPT levels was noted in 2 patients and leukopenia in 1 owing to changes in clinical examination levels.<BR>Because of the small number of cases treated, it may be too early to define the safety and efficacy of 6315-S. Nevertheless, we conclude that this drug is useful for the treatment of chronic and complicated UTI and as a prophylaxis.