B-Phycoerythrin (B-PE), a fluorescent protein from phycobiliprotein family, is isolated from cyanobacteria and eukaryotic algae. Its primary absorption peak is at 545 nm with a secondary peak at 563 nm. B-PE consists of a, b and g subunits and is present as (ab)6g. B-PE and the closely related R-PE are the most intensely fluorescent phycobiliproteins having orange fluorescence. They are significantly brighter and more photostable than conventional organic fluorophores. B-PE labeled streptavidin, primary and secondary antibodies have been widely used in applications such as flow cytometry and multi-color immunofluorescent staining. The CPK B – PE Labeling Kit contains SH-reactive B-PE. SMCC modified B-PE reacts with the thiol groups of target antibody without the need for additional activation, thus simplifying the conjugation protocol.