The pharmacokinetics of Mezlocillin were determined after the intramuscular injection of a single 1-gram dose in 10 subjects with normal renal function, in 10 patients with stabilized renal impairment and in 5 patients with end-stage renal disease submitted to repeated hemodialysis. In normal subjects, biological half-life, Tb ½„ was equal to 0.9 h; total clearance (Ct) to 449 ml/min/1.73 m2; renal clearance (Cr) to 263 ml/min/1.73 m2.72.2% of the administered dose was excreted in the urine within 12 h. In patients with renal insufficiency and in patients undergoing long-term hemodialysis, the serum concentration decrease was markedly slower. During a 6-hour dialysis session, 62% of the Mezlocillin present in the central compartment at the start of hemodialysis was removed. In the 25 subjects under study, a significant correlation was found between the values of Ke and those of creatinine clearance, Ccr (Ke=0.1973+ 0.0046 Ccr). This relation was used to calculate the loading doses, the maintenance doses and the dosage intervals adjusted to the degree of renal impairment, allowing assessment of useful dosage recommendatio