The Spirulina platensis phycobilisomes were isolated by sucrose density gradients ultracentrifugation, and the fluorescence emission maximum of the phycobilisomes at room temperature was at 671 nm. The effects of ionic strength and the zwitterionic detergent CHAPS on the stability of the Spirulina platensis phycobilisomes were studied by room temperature fluorescence spectrum. The phycobilisomes were stable in 1.0 mol x L(-1) phosphate buffer solution, and their fluorescence emission maximum could remain unchanged for 7 days. The fluorescence emission maximum of phycobilisomes was blue-shifted to 648 nm when the concentration of the phosphate buffer solution was diluted to 0.1 mol x L(-1) with deionized water, which suggested that the phycobilisomes had been dissociated. The phycobilisomes were readily dissociated in phosphate buffer solutions of low concentrations (< 0.6 mol x L(-1)) and the speed of the dissociation increased with decreasing the concentration of the phosphate buffer solution. The fluorescence emission maximum of the phycobilisomes in 1.0 mol x L(-1) phosphate buffer solution was blue-shifted to 648 nm when 10 mmol x L(-1) CHAPS was added into the phycobilisomes solution, suggesting that CHAPS could dissociate phycobilisomes under high ionic strength conditions. The results might be useful for isolating intact substructures of phycobilisomes.