3,3′,5,5′-Tetramethylbenzidine (TMB) is a chemical compound commonly used in biochemical assays as a substrate for colorimetric detection. It undergoes oxidation by peroxidase enzymes to produce a blue-colored product, making it valuable in various analytical applications, particularly in immunoassays and enzyme-linked immunosorbent assays (ELISA). TMB’s high sensitivity and stability make it a popular choice for visualizing the presence of target molecules, such as antigens or antibodies, in biological samples, enabling quantitative measurements and qualitative assessments in research, clinical diagnostics, and biotechnology.