Thromboxane A2 is formed mainly in platelets where we have isolated thromboxane synthase as a cytochrome P450-like haemoprotein. A rabbit antiserum prepared against this enzyme was monospecific according to Western blot analysis and was used for immunostaining of human tissues. In liver the Kupffer cells were clearly stained and in the connective tissue, histiocytes and monocytic cells were positive. In lung, alveolar macrophages contained most of the enzyme, whereas Type I and II cells were negative. Kidney mesangial cells were stained and in lymphatic tissue dendritic reticular cells as well as invading macrophages were positive. These were also seen in the crypts of tonsils and surprisingly also the epithelium. Placenta again showed staining of the Hofbauer cells and of the endothelial lining of the vessels. Umbilical cord and uterus only contained weak antigenicity in dendritic cells.