伊红（Eosin），又称曙红、酸性曙红。一种酸性染料，能染色细胞质，胶原蛋白，肌肉纤维并能在显微镜下观察到。

实际上有两种化合物名为伊红，常用的一种是伊红-Y，呈淡黄色；另一种为伊红-B，呈淡蓝色。两种化合物可相互转化。伊红-Y是荧光素的四溴衍生物而伊红-B是荧光素的二溴二硝基衍生物。

Eosin is a name of several fluorescent acidic compounds which bind to and form salts with basic, or eosinophilic, compounds like proteins containing amino acid residues such as arginine and lysine, and stains them dark red or pink as a result of the actions of bromine on fluorescein. In addition to staining proteins in the cytoplasm, it can be used to stain collagen and muscle fibers for examination under the microscope. Structures that stain readily with eosin are termed eosinophilic.