The pharmacokinetic properties of mezlocillin were evaluated in newborn infants. Mean peak and trough concentrations of drug in plasma, after 75 mg of mezlocillin per kg given intravenously, were 252 and 72 micrograms/ml, respectively, in infants who were less than 38 weeks gestation and less than or equal to 7 days old, compared with 139 and 9 micrograms/ml, respectively, in infants greater than or equal to 38 weeks gestation and greater than 7 days old. The mean elimination half-life values were from 4.5 h in preterm infants who were less than or equal to 7 days old to 1.8 h in term infants greater than or equal to 7 days old. Median peak and trough bactericidal titers of drug in plasma from neonates treated with mezlocillin were 1:8 and 1:4, respectively, against a resistant (minimal bactericidal concentration, 512 micrograms/ml) Escherichia coli strain and 1:64 and 1:32, respectively, against a susceptible (minimal bactericidal concentration, 2 micrograms/ml) E. coli strain. We propose a dosage schedule of 75 mg of mezlocillin per kg administered every 12 h to preterm (gestational age less than 38 weeks) infants less than or equal to 7 days old, every 8 h to preterm infants greater than 7 days old or term infants less than or equal to 7 days old, and every 6 h to term infants greater than 7 days old.