The behaviour of the histochemical ATPase reaction (Wachstein and Meisel) has been studied by light microscopy in the livers of man and mouse, during the course of viral hepatitis. This infection brings about a marked reduction or the disappearance of this enzymatic activity from the bile capillaries. This light microscopical observation is paralleled, in the electron microscope, by the disappearance of the phosphotungstic acid staining, which is characteristic of the plasma membrane bounding the bile canaliculus. The observations reported here point out the importance of the functional integrity of the biliary pole of the cell in determining its stainability by the two methods used here.