Allophycocyanin (APC) belongs to a family of phycobiliproteins that are well 
suited as fluorescent reagents for flow cytometric analysis, since they have a 
broad excitation spectrum, a large Stoke's shift and they fluoresce with a high 
quantum yield. The widespread use of APC has been limited by the availability of 
raw material and high cost of the purified phycobiliprotein. We have assessed 
the suitability of dry, powdered Spirulina platensis, available at health food 
stores, as an inexpensive source of APC. APC was extracted from Spirulina 
platensis by overnight treatment with lysozyme, followed by ammonium sulfate 
precipitation. APC was then separated from phycocyanin (the only other major 
phycobiliprotein in Spirulina) by elution of bound material from an 
hydroxylapatite column using an increasing continuous phosphate gradient. APC 
isolated in this manner retained its normal trimeric structure. The absorbance 
and fluorescence excitation and emission spectra of the purified 
phycobiliproteins were identical to those previously shown for C-PC and APC. APC 
can be stored concentrated at 4 degrees C, frozen at -70 degrees C, or as a 
saturated ammonium sulfate precipitate, with no subunit dissociation or change 
in spectral properties. Moreover, APC has been conjugated to monoclonal and 
polyclonal antibodies for use in multicolor FACS analysis, with the conjugated 
antibody activity remaining stable for at least 2 years. Thus, this procedure is 
a simple, cost-effective method for preparing reagents for multicolor 
immunofluorescence and flow cytometry.