Two toxins from a poisonous sample of mussels Mytilus edulis.

1. A crude preparation of toxin was extracted from a sample of mussels Mytilus edulis, part of a batch responsible for many cases of paralytic shellfish poisoning.2. The crude extract was partially purified by absorption on sodium Amberlite ion-exchange resin. Two toxins were recovered by elution from the Amberlite, and purified further by gel filtration.3. One toxin closely resembled saxitoxin in its behaviour on Amberlite and in its biological effects.4. The other toxin behaved quite differently on the Amberlite. Its molecule was small, comparable in size with saxitoxin. It was not tetrodotoxin. Its biological effects were similar, but not identical, to those of saxitoxin: it paralysed muscular contraction and inhibited conduction along nerves; it caused death of experimental animals by producing a peripheral paralysis of respiration; it did not depolarize the membrane of frog skeletal muscle fibres, but acted by preventing a stimulus from initiating a conducted action potential.5. The biological effects of the second toxin suggest that, like saxitoxin and tetrodotoxin, it is an inhibitor of inward sodium ion movement through electrically excitable membranes.