Calcein, also known as fluorexon, fluorescein complex, is a fluorescent dye with excitation and emission wavelengths of 495 and 515 nm, respectively, and has the appearance of orange crystals.

After transport into the cells, intracellular esterases remove the acetomethoxy group, the molecule gets trapped inside and gives out strong green fluorescence.

Calcein is now rarely used as a Ca2+ or Mg2+ indicator because its fluorescence is directly sensitive to these ions only at strongly alkaline pH, and thus it is not particularly useful for measuring Ca2+ or Mg2+ in cells.

This fluorescence quenching response can be exploited for detecting the opening of the mitochondrial permeability transition pore (mPTP) and for measuring cell volume changes.

[7] The calcein AM assay can be used as a model for drug-drug interactions, for screening transporter substrates and/or inhibitors; and also to determine in vitro drug resistance of cells, including samples from patients.