Two monoclonal antibodies (Mabs), 12C7 and 11G11, both directed against recombinant urate oxidase (SR 29142), were selected for their epitope specificity to develop a two-site immunoassay of urate oxidase in plasma. A quantitative recovery of urate oxidase in plasma was obtained at all the concentrations tested, and the limit of quantification was found to be 0.5 ng/mL. Intra-and interassay coefficients of variation ranged from 1.2 to 6.7% and from 3.5 to 10.8%, respectively. The specificity of the two antibodies was studied in Western-blot experiments. This assay was used successfully to determine urate oxidase pharmacokinetic parameters after intravenous injection in rats and baboons. In these two species, urate oxidase pharmacokinetics was characterized by a low clearance and a low volume of distribution without gender difference.