Alpha-naphthyl acetate esterase (ANAE) reaction of mouse and rat thymus cells was studied. The majority of cells demonstrated ANAE in the form of multiple scattered granules. A reaction of this type appeared particularly labile, was found only when smears were fixed in formalin vapours and was inhibited by NaF. ANAE staining of thymus cells from mice of different ages was studied: the percentage of cells with single-spotted ANAE reaction was found to increase with age, while cells with granular scattered activity showed a progressive slight decrease. The percentage of ANAE positive cells in spleen appeared to be lower than in thymus, and showed only minor variations in different ages. In mouse and rat thymus and bowel lamina propria clusters of lymphocytes were found showing a sickle-shaped ANAE reaction, surrounding diffusely stained large cells which, on the basis of cytochemistry and phagocytosis ability, could be identified as macrophages. Analogous cell clusters were found in human tonsils and adenoids. These cell clusters were found to be similar to those formed by Ia-positive interdigitating cells which have been previously observed in human thymus, tonsils and bowel. The role of these clusters in the education process of lymphocytes is discussed.