6-E1(R)-2-(oxo-imidazolidine-1-carboxamido)-2-phenyl-acetamido]-penicillanic acid sodium salt (azlocillin, Securopen) was given in doses of 100 and 200 mg/kg body weight to children with cystic fibrosis. After intravenous bolus infections, the serum half-life was 0.82 +/- 0.12 h after the lower dose and 0.98 +/- 0.18 h after the higher dose. This was consequent to a dose limited elimination kinetics due to limitation in both renal and non-renal processes of elimination. Upon doubling of the dose from 100 to 200 mg/kg, the total body clearance dropped from 13.93 to 5.10 1/h. Evaluation of data presented in other publications indicate that a dose limited elimination kinetics is the normal situation for azlocillin. Besides, the serum concentrations of patients with cystic fibrosis were considerably lower than seen in the healthy state. The reason is faster elimination by the renal route in cystic fibrosis.