Immunohistochemistry of thoracic aortas. Thoracic aortas were excised from rats and exposed to (B, C) or not exposed to (A) 100 μM H2O2 in vitro for 16 hours. Tissue sections were immunostained with rabbit polyclonal anti-FasL antibody, and immunoreactants were visualized with a substrate solution of 3-amino-9-ethylcarbazole. Cell nuclei were counterstained with a hemtoxylin solution. Exposure of thoracic aortas to 100 μM H2O2 increased expression of FasL on the aortic endothelium (B). Arrows point to positive FasL staining (red color in the original photomicrograph). Replacement of anti-FasL antibody with non-immune rabbit IgG resulted in negative staining (C). Photographs are representative of immunostaining of three thoracic aorta specimens Original magnification, 200×.