The specific inhibitory activity exerted by okadaic acid on protein phosphatase 2A was used to assess the presence of okadaic acid in mussels, using a commercially available protein phosphatase 2A preparation. Under the conditions used, okadaic acid inhibits the enzymatic activity dose-dependently, with an IC50 = 0.26 ng/ml (0.32 nM). The assay is accurate and reproducible. Okadaic acid was detected in concentrations as low as 0.063 ng/ml in aqueous solutions and 2 ng/g in mussel digestive glands. Thirty naturally contaminated mussel samples were submitted to the protein phosphatase 2A inhibition assay as well as to an ELISA assay and to a MTT cytotoxicity assay, with similar results. The proposed assay is sensitive, rapid and does not require expensive equipment. These characteristics make it a good candidate for employment in the routine assessment of okadaic acid shellfish contamination.