The pharmacokinetics of intravenous bolus doses of 1.0 g of mezlocillin were studied in 13 persons with normal and reduced renal functions. In renal failure a moderate increase was observed for the terminal serum half-life(t1/2 beta). This changed from a mean of 1.1 h at a glomerular filtration rate of 100 ml/min to 1.6 h at 10 ml/min. The difference was not statistically significant. The excretion of unchanged drug in urine during 24 h was reduced from a mean of 59.4% (range, 52 to 77) in subjects with glomerular filtration rate above 50 ml/min to 10% (range, 7.9 to 12.1) in two patients with glomerular filtration rate of 10 to 20 ml/min. The volume of distribution during the beta-phase, Vd,b, was 14% of the body weight. Much of the antibiotic was metabolized, and this proportion increased upon reduction in renal function.