After intravenous administration of MK-826, a new carbapenem antibiotic, the compound exhibited nonlinear pharmacokinetics in rats and monkeys. In both species, time-averaged plasma clearance (based on total concentrations) increased about 5-fold over the 10- to 180-mg/kg dose range. MK-826 was extensively plasma protein bound in rat and monkey plasma, and the extent of binding was concentration dependent at plasma concentrations achieved after administration of these doses. Rosenthal analysis of the plasma protein binding indicated that there were two classes of binding sites. The binding capacity of the primary site was comparable to the plasma albumin concentration, which suggested that this primary site consisted of a single site on albumin. The extent of binding of MK-826 to rat albumin was similar to that in whole plasma. Clearance values based on unbound concentrations appeared independent of dose from 10 to 180 mg/kg, which is consistent with saturation of protein binding as the primary cause of the nonlinear pharmacokinetic behavior.