B-Phycoerythrin (B-PE), a fluorescent protein from phycobiliprotein family, is isolated from cyanobacteria and eukaryotic algae. SMCC activated B-PE is chemically modified with SMCC. SMCC reacts with the primary amine on B-PE and introduces maleimide groups to B-PE. These maleimide groups easily react with thiol groups of target protein without the need for any additional activation, resulting in convenient conjugation of B-PE with proteins. Its primary absorption peak is at 545 nm with secondary peak at 563 nm. 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 conjugates have been widely used in applications such as flow cytometry and multi-color immunofluorescent staining.