Thiolite Blue is one of the most sensitive sensors for measuring thiol compounds. It gives a blue fluorescent adduct upon reacting with thiol compounds (such as GSH and cysteine). It can be used to quantifying the number of cysteines on a protein. We have used it to measure glutathione fluorimetrically. It has >200-fold fluorescence enhancement upon reaction with thiol-containing compounds. Thiolite Blue is an excellent replacement to mBBr (monobromobimane) due to their similar spcetral properties. Compared to mBBr, the thiol adduct of Thiolite Blue has much stonger fluorescence and absorption than that of mBBr, making it a much more sensitive thiol probe than bromobimanes. Thiolite Blue is optimized for intracellular thiol detection. It is non-fluorescent outside cells, eliminating the wash step and reducing assay background.