The distinction between lymphatic capillaries and blood capillaries in the mouse tongue was studied enzyme-histochemically by light and electron microscopy. The lymphatic walls are characterized by a strong 5'-nucleotidase (5'-Nase) activity, whereas those of the blood capillaries reveal a significantly lower or no activity. The alkaline phosphatase (ALPase) activity, on the other hand, is markedly higher in the blood capillaries than in the lymphatic capillaries. The specific reaction of 5'-Nase activity in the lymphatic capillaries is obtained by simultaneous inhibition of ALPase on incubation in a medium (Wachstein and Meisel,1957) with L-tetramisole for 5'-Nase histochemistry. The distribution and intensity of the 5'-Nase activity in the lymphatic capillaries can be adequately visualized by comparison with serial cryostat sections for histochemical detection under light and backscattered imaging scanning electron microscopes. The reaction products of the 5'-Nase activity are localized on the outer surface of the cell membrane of the lymphatic endothelial cells, whereas those in the blood capillaries reveal a weak or no reaction. The present results demonstrates satisfactory isolated visualizations of 5'-Nase activity in the lymphatic capillaries and of ALPase activity in the blood capillaries.