The pharmacokinetics of mezlocillin were examined in 8 patients with normal renal function (inulin clearance, > 80 ml/min per 1.73 m2), 32 patients with moderately reduced renal function (inulin clearances, 80 to 5 ml/min per 1.73 m2), and 12 patients maintained by hemodialysis or peritoneal dialysis because of severely impaired renal function. A single dose of 60 mg of mezlocillin per kg of body weight was infused intravenously over 30 min. Antibiotic concentrations in plasma, urine, and dialysate were determined by the agar diffusion technique. The half-life of mezlocillin increased with decreasing renal function from an average of 53 min in subjects with normal function to 165 min in oligoanuric patients. The urinary recovery of this drug in 24 h decreased from 65% at a glomerular filtration rate of 92 ml/min to 7.6% at a glomerular filtration rate of 6.7 ml/min. Volume of distribution was not changed by the renal insufficiency, amounting on the average to 22.5% of body weight. Intermittent hemodialysis or peritoneal dialysis contributed to only a minor degree to the 24-h mezlocillin kinetics. The pharmacokinetic properties of mezlocillin permit a normal dosage over wide ranges of renal insufficiency; however, when the glomerular filtration rate is below 10 ml/min, the dosage interval should be increased from 8 to 12 h.