Cytochemical and electron-microscopic studies have been carried out on leukemic monocytes and ‘hairy cells’ (HC), ‘reticulosarcoma’ (RS) cells and cells of cases of ‘reticulosis’ and ‘reticulosarcoma cell leukemia’. Additional investigations included quantitative determinations of the urinary lysozyme excretion, skin window studies, testing of the phagocytosis of ferritin by HC, and labelling of the Fc receptors on HC at the ultrastructural level. Clear evidences against any cytological relationship among leukemic HC and monocytes have been provided. Further results argued also against the frequently stressed relationship among leukemic monocytes and RS cells. Cases of ‘RS cell leukemia’ and ‘reticulosis’ had to be reclassified as lymphosarcoma cell leukemia, acute lymphatic, and myeloblastic leukemias. Besides distinct ultrastructural differences among HC, RS cells, and lymphocytes, mainly gradual differences have been noted using cytochemical methods and by evaluating the phagocytosis of ferritin particles. A further common trait of HC, RS cells, and B lymphocytes seems to be the presence of surface Fc receptors. A more precise classification instead of the diagnosis ‘reticulosarcoma’ and ‘reticulosarcoma cell leukemia’ is required, and the use of the term ‘hairy cell’ leukemia is suggested instead of the misleading term ‘leukemic reticuloendotheliosis’.