The analysis of diarrhetic shellfish poisoning toxins at the ng/g level in shellfish and plankton was achieved using ionspray liquid chromatography-mass spectrometry (LC-MS). Complex mixtures of naturally-occurring derivatives of okadaic acid and dinophysistoxins-1 and -2 were also detected and identified in mussel digestive glands and in Prorocentrum lima cultures. LC-MS facilitated the discovery of enzyme-catalyzed hydrolysis and transesterification reactions of some of these derivatives during extraction. Two improved extraction procedures were then developed: one that inactivates the enzyme before extraction, and another that takes advantage of the enzymatic action to convert all ester derivatives to parent toxins for quantitative assessment of toxicity.