Janus Green B is a basic dye and vital stain used in histology.

It is also used to stain mitochondria supravitally, as was introduced by Leonor Michaelis in 1900.

[2] The indicator Janus Green B changes colour according to the amount of oxygen present.

[3] When oxygen is present, the indicator oxidizes to a blue colour.

In the absence of oxygen, the indicator is reduced and changes to a pink colour.