In June of 1996, three family members were diagnosed as suffering from neurotoxic shellfish poisoning (NSP) as a result of eating shellfish harvested from Sarasota Bay, Florida. Urine from two of these patients and extracts of shellfish collected from the same location were analyzed by radioimmunoassay (RIA) and by receptor binding assay. Activity consistent with brevetoxins was present in both urine and shellfish extracts. High performance liquid chromatographic (HPLC) analysis of shellfish extracts demonstrated multiple fractions recognized by specific anti-brevetoxin antibodies, suggesting metabolic conversion of parent brevetoxins. Affinity-purification of these extracts yielded four major peaks of activity. One peak was identified by HPLC-mass spectroscopy (HPLC-MS) to be PbTx-3, which was likely produced metabolically from the dominant parent toxin PbTx-2. No PbTx-2, however, was detected. Other peaks of activity were determined to consist of compounds of apparent masses of [M + H]+ of 1018, 1034, and 1005. These higher masses are suggestive of conjugated metabolites, but their structures have yet to be determined. The material associated with these latter three peaks were recognized by both RIA and receptor binding assay, but they quantitated differently. This finding suggests that these metabolites react differently in the two assays, and this result may have important implications for seafood safety and regulation. We suggest these metabolites to be the true cause of NSP, and they should be taken into account during regulatory testing.