In 13 newborns and 12 prematures the pharmacokinetic parameters of 6-[(R)-2-(2-oxo-imidazolidine-1-carboxamido)-2-phenyl-acetamido]-penicillanic acid sodium salt (azlocillin, Securopen), were investigated after a single i.v. load of 50 mg/kg body weight. From the concentration-time curve an open two-compartment model could be postulated. The values of the microconstants indicate that there is a rapid diffusion from the peripheral compartment into the central one. The elimination half-life calculated from the beta-slope is 2.6-2.5 h. differences between newborns and prematures are lacking. To reach an average steady-state concentration C-infinity between 50 and 80 microgram/ml plasma, 100-200 mg azlocillin/kg body weight during 24h must be given. The accumulation rates for a dosage interval of 6 or 8 h are 0.6 and 0.45.