Xylenol orange is a fluorochrome useful in single and polychrome sequential labeling because it remains fixed to calcified tissues until removal of bone minerals. The fluorescence in xylenol orange is in contrast to the fluorescence of other known dyes thus allowing for easier identification of certain regions of a slide. In addition, the fluorescence labeling sites are the same for xylenol orange and other tetracylines such as fluoresceins and calcein blue. Further studies have shown that xylenol orange is used to detect lipid hydroperoxide in low density lipoproteins by oxidizing ferrous ions from Fe2+ to Fe3+ under acidic conditions. After oxidation of ferrous ions, a Fe3+ xylenol orange complex is formed which can be measured spectrophotometrically at 560 nm.