The pharmacokinetics of cefonicid was studied in 17 children requiring antibiotic treatment for respiratory or urinary tract infections. After informed consent had been obtained from the parents, a single dose of cefonicid 50 mg/kg/body weight was given by intramuscular injection. The mean peak serum concentration of 212.63 micrograms/ml was reached at 1.00 h, as absorption occurred at a very fast rate with a mean constant of 3.24 h-1. Mean values for half-life, apparent volume of distribution (Vz), total body clearance (CL), and renal clearance (CLR) were 3.24 h, 0.21 l.kg-1, 16.67 ml.min-1 and 13.60 ml.min-1 respectively. There was an inverse relationship between age and Vz, whereas CL and CLR were positively correlated with age. Cefonicid concentrations in urine were many times higher than the MICs of susceptible strains of bacteria. The study demonstrated that i.m. cefonicid 50 mg.kg-1 gave serum concentrations well within the therapeutic range for susceptible bacteria, and that its pharmacokinetic properties allow single daily doses to be used to treat infections in children.