This study was undertaken to determine the sensitivity and selectivity of protein biomarkers produced (expressed) in clams as a result of exposure to polyether toxins from the toxic dinoflagellate, Gymnodinium breve.  Clams (Merceneria merceneria) were exposed to live G. breve cells for periods ranging from 3 to 21 days and subjected to depuration following exposure for 14 days.  Of five possible biomarkers observed from preliminary investigations, two were consistently expressed during the replicated experiments and only one was found exclusively following toxin exposure. The results indicate that the expression of the five protein biomarkers was variable between exposure times and culture conditions. Therefore these protein biomarkers failed to serve as reliable markers of polyether toxin exposure in clams. This study was undertaken as a follow-up investigation of protein biomarkers induced in clams as a result of exposure to brevetoxins. Preliminary investigations conducted by Mote Marine Laboratory and the Medical University of South Carolina identified five individual markers produced in the siphon of M. merceneria in response to exposure to G. breve. The markers correspondence to elevated levels of brevetoxin in the clams as determined by HPLC. Four of the markers were induced by exposure to G. breve and one was extinguished.