Laboratory and clinical studies were performed on a new semisynthetic cephalosporin, cefamandole (CMD), and following results were obtained. (1) Serum concentrations and urinary recovery rates of CMD were determined after an intravenous administration of CMD 30 mg/kg in 13 children with normal renal function. In 5 of 13 children, mean serum levels after a one shot intravenous injection were 112.5 micrograms/ml at 15 minutes, 52.2 micrograms/ml at 30 minutes, 23.3 micrograms/ml at 1 hour, 4.9 micrograms/ml at 2 hours and trace at 4 hours. In other 5 children, mean serum levels after drip infusion for 1 hour were 78 micrograms/ml at 30 minutes, 59 micrograms/ml at 1 hour, 9.8 micrograms/ml at 2 hours and trace at 4 hours, after the onset of drip infusion. In the remaining 3 children who received CMD by drip infusion for 2 hours, mean serum levels were 24.3 micrograms/ml at 30 minutes, 35.3 micrograms/ml at 1 hour, 30.2 micrograms/ml at 2 hours, 5.3 micrograms/ml at 3 hours and 1.5 micrograms/ml at 4 hours after the onset of drip infusion. Urinary recovery rates in 5 children were 154.7%, 98.3%, 93.2%, 111.8% and 66.9%, respectively, during 8 hours. (2) CMD was administered to 40 patients with various infections (acute U.T.I. 8, acute angina lacunaris; 2, acute bronchitis; 5, cervical purulent lymphadenitis; 2, post-measles bronchopneumonia; 3, acute bronchopneumonia; 18, pyothorax; 2, S.S.S. syndrome; 1) by one-shot intravenous injection at a dose of 40-120 mg/kg per day. The clinical efficacy rate was 92.5% and bacteriological efficacy rate was 79.2%. (3) As the side effect of CMD, eosinophilia was observed in 1 case, rash and elevation of GOT and GPT in 1 case, and proteinuria in 1 case.