SUMMARY: Cytotoxic material (‘prymnesin’) excreted into the growth medium by the phytoflagellate was partially purified and crudely characterized. It proved to be a highly potent haemolysin; its solubility properties and some other characteristics are those of a lipid, perhaps a saponin. The high haemolytic activity which characterized the toxin was often also accompanied by a high ichthyotoxic activity. A method of locating haemolytic activity in a paper chromatogram is described. Fish were killed not by prymnesin alone, but were rendered highly susceptible to prymnesin poisoning by a range of cofactors: streptomycin sulphate, calcium chloride, magnesium chloride. Ichthyotoxicity in cofactor + prymnesin systems conformed within a wide concentration range to the relationship: = , where is a prymnesin concentration, a minimum concentration of cofactor which elicits a lethal effect in presence of , and is a cofactor-specific constant. The recognition of this relationship has enabled a sensitive assay to be developed. Practical implications are discussed and the hypothesis proposed that prymnesin acts on fish by inducing a state of abnormal permeability of the gill.


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