Calcein is a cell staining reagent that fluorescently labels live cells and fluoresces green (Ex=490 nm, Em=515 nm). Calcein is the most suitable fluorescent probe for live-cell staining due to its very low cytotoxicity compared to other similar reagents such as BCECF-AM and CFDA, and does not inhibit any cellular functions such as proliferation and chemotaxis of lymphocytes.rnCalcein is often used in combination with dead cell fluorescent probes such as propidium iodide (PI) to perform both fluorescent double staining of live and dead cells. Propidium iodide (PI) cannot cross the membrane of living cells, but can only pass through the disordered regions of the dead cell membrane to reach the nucleus, and intervene in the cell's DNA double helix to produce red fluorescence (Ex=535 nm, Em=617 nm), so PI stains only dead cells. Since both Calcein and PI-DNA can be excited at 490 nm, both live and dead cells can be visualized with fluorescence microscopy. With 545 nm excitation, only dead cells can be observed.