Immunohistochemical staining of a midbrain area from EL case #21225. A photomicrograph is shown at the greatest dilution of anti-poliovirus antibody at which the result was positive (0.05 N. D.). In this image, it may be noted that microglia and neuropil are stained as well as neurons. This is consistent with the TEM finding of extensive damage to the microglia and neuropil in the cases of acute EL (Figure 5). In areas of the tissue section in which the neurons were not stained, the surrounding neuropil and microglia were also unstained. Black formaldehyde pigment was present that was insolvent to 1% borohydride. Sections that were similarly prepared but with the omission of the primary antibody, showed no staining of neural, vascular or connective tissue elements, reflecting the absence of brain biotin. For highly sensitive immunohistochemical staining the sections were not counterstained and thus tissue structure was not visible in the control slides. The cytoplasm and nucleus of the neurons in the EL cases were positively stained to the same intensity, hence the nuclei were not clearly distinguished from the cytoplasm of the cells.