Allophycocyanin appears to be the pigment through which energy trapped by phycobiliproteins is funneled to the chloroplast lamellae. Isolated, intact phycobilisomes from Porphyridium cruentum have a maximum fluorescence emission peak at 675–680 nm when excited at 545 nm. Upon dissociation, when the energy transfer is interrupted the 675–680-nm peak declines. Excitation at 435 nm produced no significant fluorescence at this wavelength.