The pharmacokinetics of mezlocillin after intravenous infusion of 5 g over 30 minutes was compared in 15 patients divided into 3 groups. Group I patients had normal renal function, group II patients had moderate chronic renal impairment and group III patients were under haemodialysis. Plasma mezlocillin levels were measured by the microbiological method. Plasma kinetic values were calculated from a linear two-compartment model, using global estimation of macroconstants by the non-linear least square method. The elimination half-life was 1.19 +/- 0.107 h in group I patients 1.98 +/- 0.19 h in group II patients and 2.34 +/- 0.11 h in group III patients. Renal impairment did not significantly alter the apparent volume of distribution but influenced the total plasma clearance which was 139 +/- 9 ml/min, 87 +/- 9 ml/min and 61 +/- 8 ml/min in groups I, II and III patients respectively. There was a slight decrease of V1 (central compartment) and a noticeable increase of V2 (peripheral compartment), depending on the degree of renal impairment. The total volume of distribution (V1 + V2) remained relatively constant between 12 and 14 litres. Homogeneous results were obtained in each group of patients, thus confirming the regularity of the mezlocillin kinetic model at the dose administered. This should enable dosage adjustment according to renal function, as assessed by laboratory criteria.