A method for quantitatively assaying the presence in marine samples of diarrhetic shellfish poisoning (DSP) toxins having phosphatase inhibitory activity, said method comprising:
- preparing a marine extract;
- fractionating said marine extract to obtain a plurality of extract fractions, at least one of said fractions containing the toxin to be assayed;
- selecting the extract fraction containing the toxin to be assayed;
- incorporating at least one protein phosphatase inhibited by said toxin and a labelled physiological substrate acted upon by said phosphatase to said extract fraction;
- and
- quantitatively measuring the amount of toxin present in said extract fraction by means of said label and said substrate through the measurement of the ability of said fraction to inhibit catalysis of said labelled substrate by said protein phosphatase.