An approach involving chemical and biological techniques was taken for the detection and quantification of the marine toxin okadaic acid (OA) in mussels from Thermaikos Gulf, Greece, during a 4-month DSP episode that occurred in 2002. Samples were analyzed using the mouse bioassay, high performance liquid chromatography (HPLC) with fluorimetric detection and an enzyme-linked immunosorbent assay (ELISA). Okadaic acid was quantifiable at three different sites of Thermaikos Gulf reaching a maximum concentration of 36 microg/g hepatopancreas. High correlation was revealed between results derived from HPLC and ELISA (R2=0.998), while 91% consistency between HPLC and the mouse bioassay results was observed.