R-Phycoerythrin (R-PE), a fluorescent protein from phycobiliprotein family, is isolated from red algae. Its primary absorption peak is at 565 nm with secondary peaks at 496 and 545 nm. The broad excitation spectrum provides the advantage for multi-color immunofluorescent staining or cell sorting. R-PE consists of a, b and g subunits and is present as (ab)6g. R-PE and the closely related BPE are the most intensely fluorescent phycobiliproteins having orange fluorescence. They are significantly brighter and more photostable than conventional organic fluorophores.