Ehrlichs reagent or Ehrlich reagent is a reagent containing p'-dimethylaminobenzaldehyde (DMAB) and thus can act as an indicator to presumptively identify indoles and urobilinogen. Several Ehrlich tests use the reagent in a medical test; some are drug tests and others contribute to diagnosis of various diseases or adverse drug reactions. It is named after Nobel Prize winner Paul Ehrlich who used it to distinguish typhoid from simple diarrhoea.
The Ehrlich reagent works by binding to the C2 position of two indole moieties to form a resonance stabilised carbenium ion compound.