R-Phycoerythrin (PE) is isolated from red algae. Its primary absorption peak is at 565 nm with secondary peaks at 496 and 545 nm. The relative prominence of the secondary peaks varies significantly among R-PEs from different species. PE has three types of subunits: alpha (20,000 daltons), beta (20,000 daltons) and gamma (30,000 daltons). The molecular weight of intact PE has been found to be about 240,000 daltons. The alpha subunit of PE contains only the phycoerythrobilin (PEB) chromophore, while beta and gamma subunits contain both PEB and phycourobilin (PUB). Variability in the absorption spectra of PEs from various species reflects differences in the PEB/PUB ratio of the subunits. PE and closely related B-PE are the most intensely fluorescent phycobiliproteins, with quantum efficiencies probably in excess of 90%, and its orange fluorescence is readily visible by eye in any moderately concentrated solution.

For an example antibody conjugation protocol, please see here.