Mezlocillin was administered as an intravenous bolus injection to eight children, aged 4-15, requiring antibiotic therapy; the dose was either 75 mg/kg or 37.5 mg/kg. All of the patients had advanced renal insufficiency. Five of the children required chronic intermittent hemodialysis. The microbiological method of Grove and Randall was used for the quantitative determination of mezlocillin in the serum. Ten minutes after the injection, the concentration of mezlocillin in the serum was approximately 90 mg/l and 40 mg/l, respectively. The determinations 30 and 60 minutes after the injection showed that the mezlocillin concentrations had fallen rapidly to approximately 30 mg/l and 20 mg/l, respectively. The mezlocillin concentrations were still approximately 2 mg/l and 1 mg/l, respectively, 12 hours after administration. For the period between 10 and 60 minutes, the half-lives were between 33 and 57 minutes. In general, we can conclude that a single bolus injection of 75 mg/kg will result in mezlocillin concentrations in the serum of uremic children which allow us to expect good antibacterial efficacy, at least for the first few hours following the injection. On the basis of our studies, half the dosage does not appear to be sufficient.