The test based on morphological changes in KB cells was assayed with different toxins. Only lipophilic inhibitors of protein phosphatases, such as okadaic acid or calyculin A, induced visible changes in cell morphology. The activity of contaminated mussel extracts on KB cells was evaluated comparatively by direct interpretation of morphological changes and by a colorimetric method estimating the number of viable cells after staining. The latter technique revealed interferences (not detected by the former) with mussel cytotoxins. These results show that the technique, based on determination of the minimal active concentration of toxic extracts inducing morphological changes, is more specific, faster and preferable to the determination of IC50 for the detection of protein phosphatase inhibitors in shellfish.