Using the monoclonal antibody Tü 300 we localized thromboxane synthase, a secondary enzyme of the arachidonic acid cascade, employing the alkaline phosphatase anti-alkaline phosphatase method and indirect double labelling immunofluorescence in frozen sections of human tissues. Aside from platelets, the source of the antigen, all cells of the mononuclear phagocytic system were positive, including epithelioid cells and associated giant cells, starry sky macrophages, dendritic cells of T-cell areas, Langerhans cells and Kupffer cells. In addition, some epithelial cells such as epithelia of tonsillar crypts, reticular epithelia of the thymic cortex and ductular epithelia in liver, pancreas, female breast and salivary glands showed occasional focal reactivity for thromboxane synthase. We suggest that the mAb Tü 300 is a key marker for the macrophage system and the thromboxane generating system in normal and pathological conditions. It may detect functional activities of as yet unknown significance in some specialized epithelial cells.