Since January 1993, neurological symptoms and rapid deaths (5 to 10 min) were typically observed in the mouse bioassay of acetone extracts of digestive glands from Arcachon and Toulon (France) during the winter season. It was assumed initially that a new lipophilic toxin was present because tests using the AOAC mouse bioassay for paralytic shellfish toxins on acid extracts of whole shellfish meat were negative, no known lipophilic toxins were detected and no toxic phytoplankton species were observed in the area during the poisoning events. In this study, however, preparative isolation of the toxic factor from toxic mussel digestive glands has revealed the presence of paralytic shellfish toxins, the principal ones being gonyautoxins-2 and -3 at Arcachon and gonyautoxins-1, -4, -2 and -3 at Toulon. The toxin concentrations recorded were below levels harmful to consumers and therefore represent a false positive in the mouse bioassay for lipophilic toxins based upon acetone extraction. The origin of the toxins remains to be determined. Copyright © 1999 John Wiley & Sons, Ltd.