Sufficient amounts of C1/C2 and GTX2/3 have been produced for metabolism studies from a Hong Kong strain of Alexandrium tamarense that produces C1/C2 in high yields. The toxic dinoflagellate was cultured in Fernbach flasks in artificial seawater K-medium at 23C with 5000 lux and a 16/8 hour light cycle. Our results indicate that large quantities of C-toxins are present extracellularly in the culture medium of batch cultures of Alexandrium tamarense. Intracellular toxins were extracted by sonication of harvested cells. Extracellular toxins were extracted and concentrated by passing the spent culture medium through a strong anion exchange column. Toxicity throughout the study was followed using the mouse neuroblastoma assay. Toxin content was analyzed using HPLC-FLD. Intracellular concentrations of C1/C2 were found to be approximately 16 fmol/cell at late log phase with a comparable proportion of toxins present in the culture medium. In this case, recovery of intracellular toxins alone would underestimate the total toxin production by up to 50%. Combining both toxin sources, in which one half of the C-toxins were hydrolyzed under optimized acidic conditions to GTX2/GTX3, led to the recovery of mg quantities of purified toxins for further studies. These findings indicate that: 1) spent culture medium is a readily available source of toxins and 2) toxins present in the culture medium should not be ignored in determining the total toxin production by Alexandrium.