A new spectrofluorometric method for cellular prion protein (PrP(C)) was developed based on the regulation of N,N'-bis[3,3'-(dimethylamino)propylamine]-3,4,9,10-perylenetetracarboxylic diimide (DAPER) fluorescence. As a perylene derivative, DAPER emits strong fluorescence in the form of free monomer in aqueous medium, but not in the form of aggregates. In this contribution, we found that the aptamer of PrP(C) could induce the aggregation of DAPER, and the bright fluorescence of DAPER was completely quenched. The quenched fluorescence, however, was recovered if PrP(C) was further added, which was ascribed to the specific binding of PrP(C) to its aptamer and the releasing of free DAPER monomers. This signalling mechanism makes it possible to detect PrP(C) by fluorescence spectroscopy. The assay allows the selective determination of PrP(C) in aqueous solution with high sensitivity and exhibits a good linear range from 0.4 to 1.6 nmol L(-1). Moreover, this probe can be applied to monitor the level of PrP(C) in human urine samples with satisfactory results.