Monoclonal antibodies against prostaglandin H (PGH) synthase have been used to precipitate cyclooxygenase and peroxidase activities from detergent-solubilized preparations of ram seminal vesicle microsomes. Approximately 85% of the solubilized cyclooxygenase activity was precipitated using an excess of anti-PGH synthase antibody; under similar conditions, immunoprecipitation of 60% of the diphenylisobenzofuran peroxidase, 75% of the phenylbutazone peroxidase, and 50% of the epinephrine peroxidase activities occurred. In contrast, less than 10% of the cyclooxygenase or peroxidase activities could be precipitated with nonimmune, control antibody preparations. These data indicate that the hydroperoxidase activity of PGH synthase is the major peroxidase activity catalyzing the co-oxidation of xenobiotics in ram seminal vesicle microsomes.