A new cephalosporin cefsulodin (CFS) was studied basically and clinically and the following results were obtained. 1. The serum levels of 25 mg/kg of CFS administered intravenously were 39.5 mcg/ml after 30 minutes, 22.6 mcg/ml after 1 hour, 11.6 mcg/ml after 2 hours, 6.0 mcg/ml after 4 hours and 2.1 mcg/ml after 6 hours. The half life from serum was 84 minutes. 2. Clinical response on 4 cases of Pseudomonas aeruginosa infections were all good. 3. The slight elevations of GOT, GPT were observed by the drug administrations in 1 case. From the above results, CFS was effective drug to P. aeruginosa infections by intravenous administration of 25 mg/kg of CFS.