Ticarcillin was evaluated in 82 neonates and young infants with suspected sepsis and in 16 older children with chronic Pseudomonas infection of the mastoids. The infants also received kanamycin. Individual ticarcillin doses of 75 or 100 mg/kg were given every four, six, or eight hours by intramuscular injection or by a 30-minute intravenous infusion. Mean plasma concentrations one hour after a dose were from 125 to 189 microgram/ml, depending on dosage, age, and maturity. Mean plasma half-lives were approximately 5 hours in the first week of life, 2 hours in infants from 1 to 8 weeks, and 0.9 hours in older children. Volume of distribution was approximately twice as great in infants as in children, and plasma clearance rates correlated inversely with age. Limited efficacy data suggest that ticarcillin is a suitable alternative to ampicillin or carbenicillin, when given concurrently with an aminoglycoside, for newborn infections. When given for several days before mastoidectomy and tympanoplasty, ticarcillin sterilized the mastoids in the majority of patients. A new dosage schedule for ticarcillin in pediatric patients is proposed.