Fundamental and clinical studies on cefpiramide (CPM), a new semisynthetic cephalosporin, were made and the following results were obtained. The antibacterial activities of CPM against clinical isolates were almost similar to those of conventional cephems except for Pseudomonas aeruginosa. The antibacterial activity of CPM against P. aeruginosa was excellent and superior than those of the others. Ten or twenty mg/kg of CPM was given intravenously at one shot to 11 cases. The mean serum levels of CPM reached 231 micrograms/ml at 15 minutes, 119 micrograms/ml at 30 minutes, 88 micrograms/ml at 1 hour, 65 micrograms/ml at 2 hours and 33 micrograms/ml at 6 hours after administration at a single dose of 10 mg/kg, respectively with the half-life of 3.42 hours. In case of 20 mg/kg, the mean serum levels attained 306 micrograms/ml at 15 minutes, 245 micrograms/ml at 30 minutes, 160 micrograms/ml at 1 hour, 118 micrograms/ml at 2 hours and 66 micrograms/ml at 6 hours respectively after administration with the half-life of 5.20 hours. CPM was given intravenously to 12 patients with various bacterial infections. The clinical effects were excellent in 5 cases, good in 6 cases and poor in 1 case and the effective rate was 92%. No side effect was observed in all cases.