DSP human toxication outbreaks with detection in responsible bivalves of very low content of toxins, measured with current methods, have called attention for possible deficience on total toxin extraction. The presence of acyl esters of okadaic acid and DTX2 has been confirmed in several species of Portuguese shellfish by HPLC with fluorometric detection. In certain instances, the content of esters has surpassed 50% of total DSP toxins found. The fluorometric method of Lee, currently used by our laboratory to confirm the presence of DSP toxins, does not detect these toxins. The need to introduce a hydrolysis step in sample preparation for HPLC is thus emphasized due to results obtained. Experiments were also carried out on sample preparation for mouse bioassay.