The thymus was the site of a true histiocytic lymphoma. Immunohistochemical demonstration of lysozyme and alpha-1-antitrypsin in the tumor cells indicated the histiocytic nature of the tumor. Five fetal and five adult thymus glands were studied for the presence and distribution of true histiocytic cells. Histiocytes were demonstrated predominantly in the connective tissue septa and constituted approximately 2% of cells in these thymuses. Histiocytes are therefore a normal constitutent of the thymus, which may become the primary site of true histiocytic lymphoma.