Janus Green B, also known as Kenna Green, is a living stain that is used exclusively for staining mitochondria and nuclei, and also for determining cell survival. The principle of staining is that it binds to cytochrome C oxidase in mitochondria, resulting in a blue-green color.

Janus Green B Stain (1%) is prepared from Janus Green B and Ringer's solution. It is important to keep the specimen active during the staining process and to manipulate it quickly. As the cells die or begin to die, the cytoplasm and nucleus are stained with varying degrees of green as the enzyme is inactivated. It is important to be accurate and quick in taking the specimen; during staining, the surface of the tissue block is exposed to the staining solution so that the enzymes of the intracellular mitochondria can carry out oxidation sufficiently to keep the dye in an oxidized state.