TMB (3,3´,5,5´ tetramethylbenzidine) is a substrate for horseradish peroxidase (HRP).

The 1-Step™ Turbo TMB-ELISA is a complete solution that contains TMB and a stabilised hydrogen peroxide. TMB has an absorption maximum at 285 nm before oxidation. After oxidation, the maxima are 370 and 652 nm. The blue solution can be measured at 652 nm. However, upon addition of the stop solution (ie. acid) a yellow solution results that can be measured at 450 nm and has a higher sensitivity than the measurement of the blue solution.