A single intramuscular dose of 100 mg carbenicillin/kg produced measured peak concentrations of approximately 150 to 175 mug/ml in newborn infants. The carbenicillin serum half-life of 1.0 hr in normal adults was prolonged to 2.7 hr in infants of normal birth weight and to 4.0 hr in babies of low birth weight in the first week of life. One-third of a dose administered to babies of low birth weight and two-thirds of that given to babies weighing over 2,500 g was excreted by the kidney in 12 hr. Postadministration 6-hr urinary carbenicillin concentrations averaged 1,399 mug/ml after an intramuscular dose of 50 mg/kg and 2,689 mug/ml after a 100 mg/kg injection. Although carbenicillin is excreted by both glomerular and tubular mechanisms in adults, serum half-life and urinary carbenicillin excretion correlated well with creatinine clearance. Recommendations for dosage of carbenicillin and intervals of administration in the neonatal period are made based upon mathematical predictions from pharmacokinetic data.