B-PE (B-Phycoerythrin), a fluorescent protein, belongs to the phycobiliproteins, a family of highly soluble and fluorescent proteins derived from cyanobacteria and eukaryotic algae. B-PE is made of α, β and ? subunits and is present as (αβ)6?. It has absorption bands with peaks at 545 nm (εM 2.41 X 106 M-1cm-1), and 563 (εM 2.33 X 106 M-1cm-1) and has maximal emission at 578 nm. B-PE and the closely related R-PE are the most intensely fluorescent phycobiliproteins, which is 10-20 times brighter than conventional organic fluorophores. Additionally, phycobiliproteins are more photostable than their organic counterparts. B-PE labeled streptavidin is widely used in flow cytometry, live cell staining, and multi-color immunofluorescent staining.