Abstract The heteronemertine Cerebratulus lacteus synthesizes a family of four structurally homologous, crustacean-selective polypeptide toxins (B-toxins) which affect action potential generation in nervous issue. The most abundant homolog, toxin B-IV, is completely inactivated by reduction of its four disulfide bands. Reduction is also accompanied by loss of secondary structure. Reoxidation of the reduced protein may be catalyzed by the oxidized forms of glutathione or dithiothreitol. Secondary structure and toxicity are recovered in parallel; the terminally reoxidized protein regains approximately 75% of both the neurotoxicity and the secondary structure of native B-IV.