Pharmacokinetics and clinical studies on cefsulodin (CFS) were conducted in neonates. 1. MIC's of CFS, sulbenicillin and gentamicin (GM) were determined using 7 strains of Pseudomonas aeruginosa clinically isolated from neonates and maintained as stock cultures. CFS was found to be nearly as active as GM. 2. When CFS 20 mg/kg was administered to a 12-day-old neonate by intravenous bolus injection, serum concentrations were 8.7 micrograms/ml before administration and 51.7 micrograms/ml at 30 minutes, 44.4 micrograms/ml at 1 hour, 38.6 micrograms/ml at 2 hours and 11.1 micrograms/ml at 6 hours after administration. The half-life was 2.5 hours. 3. CFS was administered alone or combination with other drugs to 3 neonates. The drug was clinically effective in 2 cases and slightly effective in another. Bacteriologically, one case was rated as decreased, another as replaced, and the remaining one as unchanged. 4. Neither side effects nor abnormal laboratory values attributable to CFS were found.