Polyclonal and monoclonal antibodies (MoAbs) were raised against human platelet thromboxane (Tx) synthase. Neither the antiserum nor the MoAbs inhibited the enzyme activity significantly. Three MoAbs, Tu 300, Kon 6, and Kon 7, were purified and further characterized. They are monospecific as shown by activity precipitation or Western blot analysis, and recognized different epitopes on Tx-synthase. Tu 300 could precipitate the enzyme and recognized conformational epitopes, whereas Kon 6 and Kon 7 only reacted in Western blots. Antibody Tu 300 can be used in immunohistology but shows no crossreactivity with Tx- synthase from other species. In human lung tissue staining with peroxidase, coupled Tu 300 was only found in alveolar macrophages.