Prussian blue is not a dye but an inorganic pigment containing ferric ferrocyanide. Procedures have been publishedfor the demonstration of non-hemoglobin, cytoplasmic iron, according to Sundberg and Broman,and for the demonstration of hemosiderin according to Highmanand from Lille and Fullmerbased the work originally done by Perle in 1867.Prussian Blue is formed in situ in both of these methods. In combination with iron particles such as superparamagnetic iron oxide nanoparticles, Prussian blue may be used for tracking of transplanted cells such as mesenchymal stem cells.Prussian blue may also be used to differentiate slow versus fast cycling cells by methods that monitor iron oxide particle labeling dilution thru cell division.