ChEBI: Fluorescein (lactone form) is a xanthene dye that is highly fluorescent, detectable even when present in minute quantities. Used forensically to detect traces of blood, in analytical chemistry as an indicator in silver nitrate titrations and in microscopy. It has a role as a radioopaque medium and a fluorescent dye. It is a xanthene dye, a gamma-lactone, a polyphenol, an oxaspiro compound, a member of 2-benzofurans and an organic heteropentacyclic compound. It is functionally related to a fluorescein (acid form). A xanthene dye that is highly fluorescent, detectable even when present in minute quantities. Used forensically to detect traces of blood, in analytical chemistry as an indicator in silver nitrate titrations and in microscopy.