TMB Substrate Solution (For Western Blotting) is a single-component horseradish peroxidase (HRP) substrate for Western blotting and immunohistochemistry experiments.It contains soluble TMB (3,3',5,5'-tetramethylbenzidine), which quickly reacts with horseradish peroxidase (HRP) enzyme to produce an insoluble and stable dark blue precipitate.