We have raised antibodies against brevetoxin (PbTx-3), and established a direct competitive ELISA suitable for use on shellfish and seawater containing marine algae. The ELISA has a limit of quantitation of 0.53 ng /ml, and limit of detection below 0.1 ng/ml. The high sensitivity of the ELISA permits use with algal cultures and the direct analysis of seawater. The ELISA has great potential for monitoring bloom dynamics and for identification of toxic dinoflagellates. Extracts of cellular material from cultured algae were prepared by centrifugation, followed by extraction of the cell pellet with methanol. These extracts were diluted for ELISA. Brevetoxin was detected in a number of isolates. Medium was also harvested from Gymnodinium breve cultures by aspiration, and gentle centrifugation to remove the cells. An aliquot of medium was analysed by ELISA. Brevetoxin was detected at relatively high levels in the culture medium of a number of high producing isolates (no less than 20% of the toxin detectable in the pellet on a \'per cell\' basis). The detection of brevetoxin at relatively high levels in the culture medium was rather unexpected. The ability to detect brevetoxin in cultured isolates opens the door to many areas of research, the most immediate benefit is the potential to characterise Gymnodinium species, and perform a chemo-taxonomic study. Of interest to regulators will be the ability to screen a wide range of algal cultures and taxa for the detection of toxin producing species. A number of the incidents of NSP toxin related closures in New Zealand have occurred in areas where Gymnodinium species were not detected.