The pharmacokinetics and safety of ceftazidime (25 mg/kg twice daily intravenously or intramuscularly) were determined in 41 young, premature neonates who were clinically infected and would otherwise have received gentamicin plus penicillin. Ceftazidime was assayed in 46 series of blood samples by HPLC. Blood was collected before, during and after treatment for analysis of biochemical and haematological factors. Faecal specimens were examined for the presence of Clostridium difficile and its toxin. Although the peak concentration following iv administration was higher (77 +/- 8 mg/l) than that following im injection (56 +/- 7 mg/l), satisfactory serum levels were maintained throughout the dosage interval using either route. Mean pharmacokinetic profiles following both routes are reported. Postnatal age was the most important factor governing total body clearance (P = 0.0001) and serum half life (P = 0.001). The biochemical and haematological status of the majority of babies remained unaffected by therapy and there was no increase in the incidence of Cl. difficile isolation from stools. Ceftazidime is a safe and well tolerated drug for use in the treatment of neonates. 25 mg/kg administered twice daily results in adequate serum levels in babies during the first two weeks of life.