Basic and clinical investigation on the intravenous administration of sulbenicillin in moderate dose (510g daily) was carried out to evaluate its clinical effect in systemic infections due to gram-negative bacilli. The following results were obtained. (1) In human subjects received 5 g intravenous drip infusion, the peak blood levels were found at the end of infusion. In 6 cases with normal renal function (Ccr greater than or equal to 70ml/min.) the peak blood level was 181 mcg/ml on the average and the half-life 1.1 hours, while in 3 cases with impaired renal function (Ccr less than 70 ml/min.) the peak level 216 mcg/ml and the half-life longer than 2 hours. The height of the peak level seemed to be subjected to the duration of infusion. The renal excretion of sulbenicllin was 55.2% on the average both in cases with normal and impaired renal functions. (2) Sulbenicillin, 510g daily divided in 2 doses, was administered to 15 cases including 6 cases with acute pyelonephritis, 3 with acute cystitis, 3 with biliary tract infection, 2 with respiratory tract infection and 1 with acute prostatitis. All the cases except 3 cases with acute pyelonephritis had underlying diseases. Escherichia coli was isolated from 10 cases, Klebsiella from 2, Pseudomonas aeruginosa from 1, and unidentified gram-negative bacilli from 1. Eleven cases responded to the treatment, but 4 cases failed. In 11 cases with susceptible bacteria, 8 cases responded bacteriologically (2 cases recurred), and 3 cases failed to respond. A case with biliary tract infection due to E. coli did not respond to 5 g daily treatment, but responded to 5 g twice daily. Two cases due to organisms which were not inhibited by 200mcg/ml in vitro did not respond to the treatment. (3) A moderate decrease in red blood cell number and hemoglobin content was observed in one case. A transient increase in transaminase and alkaline phosphatase level was observed in other cases.