A stable and highly sensitive 3,3',5,5'-tetramethylbenzidine-based substrate reagent for enzyme-linked immunosorbent assays.

3,3',5,5'-Tetramethylbenzidine (TMB) is a widely used chromogen for horseradish peroxidase-based detection systems because it yields reaction products with high absorption coefficients and lacks carcinogenicity. Unfortunately, TMB is labile and poorly soluble in aqueous buffers and such solutions must be freshly prepared before each experiment. Moreover, substrate depletion can occur under assay conditions. To overcome these problems we have developed a two-component TMB substrate system which has a lower detection limit and is more sensitive than many commercially available TMB reagents when compared in microtiter plate enzyme-linked immunosorbent assays. Both components of the substrate system are stable for at least 1 year at 4 degrees C and the usual discoloration of TMB stock solutions is prevented by the addition of a stabilizer that decomposes upon mixing of the two components.