The search for new protein phosphatase inhibitors in shellfish contaminated by toxin-producing dinoflagellates generally relies on preliminary separation techniques followed by biological tests. To detect such substances without purifying them initially, we developed an approach based on a correlation of the results of two different analytical techniques applied to toxic extracts: high-performance liquid chromatography after derivation of the toxins and the cell morphology transformation assay on KB cells. Application of this protocol to stored frozen mussels showed a decrease in okadaic acid concentration during storage, with formation of degradation derivatives, some of which possessed notable protein phosphatase inhibition activity.