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A peroxidase-linked immunoassay of the sandwich type was developed for a quantitative determination of the amount of human cyclooxygenase. Two species of monoclonal antibodies (hPES01 against the human enzyme and PES-5 against the bovine enzyme) were utilized, which recognized different epitopes on the cyclooxygenase of human platelets. The peroxidase activity of the immunoprecipitate was correlated with the amount of cyclooxygenase. The enzyme immunoassay was applied to platelets from 15 normal subjects and a clinical case of platelet cyclooxygenase abnormality with a prolonged bleeding time. Almost the same level of immunoreactive protein was found in platelets of both normal subjects and the patient However, the solubilized enzyme from the patient's platelets did not transform arachidonic acid to prostaglandin H2 (PGH2) while thromboxane production from PGH2 was observed at a normal level.