Serum and urinary levels of cefotaxime after either intramuscular (i.m.) or intravenous (i.v.) administration were determined in 6 normal human subjects. By the i.m. route the serum half-life was 80 minutes. The peak serum concentration, at 30 minutes, reached 12 and 24 mcg/ml after injection of 500 mg and 1 g. Concentrations in the urine were 500 mcg/ml within 6 hours after an injection of 500 mg; they reached 1000 mcg/ml after an injection of 1 g. Urinary elimination amounted to 50% during the first 6 hours; it reached 60% during 24 hours. By the i.v. route, the serum half-life was 40 minutes. The concentration in the serum was more than 100 mcg/ml at the fifth minute after an injection of 1 g. Concentrations in the urine were 1300 mcg/ml within 4 hours after injection of cefotaxime. The urinary elimination amounted to 60--64% during 12 hours. Intravenous infusion of 1 g during 4 hours gave a serum level, from the second hour, approaching 15 mcg/ml. The concentration in the urine was 600 mcg/ml within 4 hours. The urinary elimination amounted to 50% in 12 hours. Protein binding of cefotaxime ranged from 36 to 38%.