3,3’,5,5’-Tetramethylbenzidine

3,3’,5,5’-Tetramethylbenzidine or TMB is a chromogenic substrate used in staining procedures in immunohistochemistry as well as being a visualising reagent used in enzyme-linked immunosorbent assays (ELISA). TMB is a white crystal powder which forms a pale blue-green liquid in solution with ethyl acetate.

In solution, TMB forms a blue product when allowed to react with peroxidase enzymes such as horseradish peroxidase. The resulting colour change can be read on a spectrophotometer at a wavelength of 370 or 655 nm. Completion of the reaction results in a yellow coloration which may be read at 450 nm. The reaction can be halted by addition of acid or another stop reagent.

Material Safety
TMB should be kept out of direct sunlight as it is photosensitive. It should also be handled as a mutagen.