Abstract. Oral cephalexin was used in 45 children, aged 3 weeks to 14 years in a medium dose of 27.4 mg/kg repeated every 6--8 hrs in 19 cases of pyelonephritis, 11 cases of pulmonary infection and 15 cases of various infections for an average of 14--21 days. Clinical and bacteriological results were excellent in UTI, favourable in the other groups, as far as to be objectivatcd. During the first hours of therapy blood and urine concentrations were measured. 1VIaximum concentrations in serum after a single dose were reached by children older than 1 year after 1 hr with 25.15 meg/ml, in infants only after 2 hrs with only 9.82 mcg/ml. Urine concentrations exceeded 1000 mcg/ml in 38 out of 42 patients. WBC and neutrophils were disminished, SGOT, SGPT and blood urea elevated, but remained within normal limits.