Two areas of research on botulinal toxin are reviewed: (1) isolation and characterization of the toxin molecule and (2) the mechanism by which the toxin acts to paralyze transmission by cholinergic nerves. The various molecules of botulinal toxin (types A, B, D, E, and F) have molecular weights of ∼150,000. The toxins are composed of two subunits with molecular weights of ∼100,000 and ∼50,000, respectively. The subunits are linked by one or more disulfide bonds. The large-molecular-weight substance (∼150,000) is fully neurotoxic; neither subunit possesses neurotoxicity. Toxin-induced paralysis of cholinergic nerves involves three steps: (1) an initial binding step that involves an external receptor; (2) a translocation step during which the toxin molecule, or some portion of it, moves through the nerve membrane; and (3) a paralytic step during which the release of acetylcholine is blocked.