The neurotoxins produced by Clostridium botulinum are the most potent acute toxins known and are the causative agents of the neuroparalytic disease botulism. The toxins act primarily at peripheral cholinergic synapses by blocking the evoked release of the neurotransmitter acetylcholine. There are seven distinct serotypes of toxin. All are polypeptides of Mr about 150 kDa that have similar structure and pharmacological action. In their most active forms the toxins exist as dichain molecules in which a heavy (H) chain is linked by disulphide bonding to a light (L) chain. The H chain is believed to be associated with the highly specific and avid binding of toxin to the motor nerve end plates and also with the process of internalisation of the toxin. The toxic activity appears to be associated with the L chain which blockades the calcium-mediated release of acetylcholine, probably by interfering at the molecular level with the mechanisms whereby neurotransmitter-containing vesicles merge with the plasmalemma. The type A toxin is now used therapeutically to treat a variety of conditions involving involuntary muscle spasm. The therapeutic toxin is a neurotoxin-haemagglutinin complex isolated from cultures of C. botulinum. A controlled manufacturing process has been developed for the therapeutic toxin which is specially formulated to give a freeze-dried product having good stability.