branes; the filaments of the heart muscle fibers are also stained (Figs. l-4). An unexpected effect of chromyl chloride is that parts of the section are removed. Fig. 4 shows that the mucous granules of the surface epithelial cells of the lizard gastric mucosa appear empty, whereas in the control they are filled with material of medium density (Fig. 3). The same effect occurs on a smaller scale with the core material of what is believed to be leukemia virus [2] in the polyoma tumor of the mouse parotid gland (Figs. 5 and 6). Certain areas of extraction along the fibrous material inside the plasma membrane (Fig. 6) may indicate that the core material of the virus is being formed at these sites. It is strange that chromyl chloride should cause the removal of material from thin sections. One explanation is that the chromium stains so intensely that localized heating produced by inelastic scattering of electrons in the microscope “burns” out these areas. As indicated by Edwards [l], development of the method as a specific or semi-specific stain for viral material would be of great value.