To identify the cells containing indoleamine 2,3-dioxygenase, indirect immunofluorescence studies were performed using the monospecific antibody toward the enzyme purified to apparent homogeneity from rabbit small intestine. The antibody precipitated almost 100% of the enzyme activities of various organs or rabbit in a quantitatively similar manner, but did not cross-react with tryptophan 2,3-dioxygenase [L-tryptophan:oxygen 2,3-oxidoreductase (decyclizing), E.C. 1.13.11.11] in the liver. Immunohistochemical localization was studied using rabbit tissue because of the species-specific reactivity of the antibody. In the duodenum, indoleamine 2,3-dioxygenase-immunoreactive cels were found to be sporadically but exclusively localized in the mucosa, a large number being in the basal portion. No staining was seen in the consecutive sections treated with control immunoglobulin. Restaining within the same section using Grimelius' silver technique revealed that the immunoreactive cells coincided with the argyrophilic cells reportedly containing serotonin under physiological conditions. In the thyroid gland, the immunofluorescence was positive in the cluster of the parafollicular cells also shown to be argyrophilic, the cells proposed to have a monoamine-handling capacity specific for serotonin. These results indicate that indoleamine 2,3-dioxygenase is present in those cells rich in serotonin or active in serotonin biosynthesis and further support our previous contention that the enzyme may play an important role in the regulation of the metabolism of indoleamines and their precursors in such cells.