We developed a method to screen paralytic shellfish poisoning (PSP) toxins based on their functional activity. The assay was a fluorimetric assay by detecting changes in the membrane potential of transitional cell carcinoma of the bladder cells T24 and involved several steps: stain of T24 cells with fluorescent dye bis-oxonol, cell depolarization with veratridine, and inhibition of depolarization with PSP toxins GTX2,3 or shellfish samples containing PSP toxins. Toxic potency of the samples was evaluated by measuring toxin-induced changes in membrane potential. Within 2-100 nmol·L-1 of GTX2,3, veratridine-induced depolarization was shown to be inhibited by GTX2,3 in a dose-dependent manner. There was a linear correlation between the percentage of inhibition and toxin concentration. The PSP toxin value in shellfish obtained by this fluorescence assay was in concordance with that by the mouse bioassay, and with higher sensitivity. In conclusion, the fluorescent dye method based on changes in membrane potential was a rapid, specific, and reliable method for detecting paralytic shellfish poisoning toxins in shellfish.