Trypan blue (Fig. ) is a diazo-dye, which has played a historical role in the discovery of the bloodbrain barrier. In 1913 Edwin Goldmann, a trainee of Paul Ehrlich, injected trypan blue into cerebrospinal fluid of laboratory animals, and found that the dye stained the brain only, but not the other organs. Alternatively, intravenous injections of the dye led to staining of the body tissues, but not the brain. In 1950 it has been clarified that the staining was due to the dye-protein complexes, which could not cross the barrier .