An interlaboratory study was conducted for the determination of paralytic shellfish poisoning (PSP) toxins in shellfish. The method used liquid chromatography with fluorescence detection after prechromatographic oxidation of the toxins with hydrogen peroxide and periodate. The PSP toxins studied were saxitoxin (STX), neosaxitoxin (NEO), gonyautoxins 2 and 3 (GTX2,3 together), gonyautoxins 1 and 4 (GTX1,4 together), decarbamoyl saxitoxin (dcSTX), B-1 (GTX5), C-1 and C-2 (C1,2 together), and C-3 and C-4 (C3,4 together). B-2 (GTX6) toxin was also included, but for qualitative identification only. Samples of mussels, both blank and naturally contaminated, were mixed and homogenized to provide a variety of PSP toxin mixtures and concentration levels. The same procedure was followed with samples of clams, oysters, and scallops. Twenty-one samples in total were sent to 21 collaborators who agreed to participate in the study. Results were obtained from 18 laboratories representing 14 different countries.