3,3',5,5'-Tetramethylbenzidine

3,3’,5,5’-Tetramethylbenzidine or TMB is a chromogenic substrate for horseradish peroxidase (HRP). TMB is used in staining procedures in immunohistochemistry such as ELISA. TMB is a white crystal powder that forms a pale blue-green liquid in solution with ethyl acetate. The blue product can be read at 370 or 655 nm. The reaction may be stopped with acid, forming a yellow product that can be read at 450 nm.

TMB is degraded by sunlight and by fluorescent lights. Therefore, it must be contained in a dark or amber bottle. It cannot be stored in metallic containers.