Ricin. Computer model of a molecule of the toxic protein ricin. It comprises two amino acid chains, termed A (upper left) and B (lower right). The A-chain is toxic, inhibiting protein synthesis in cells. It is carried into cells by the non-toxic B-chain. Ricin is derived from the seeds of the castor oil plant (Ricinus communis). Less than a milligram is enough to kill an adult. Inhaling ricin causes coughing, pulmonary oedema and death. If ingested, it causes gastroenteritis, liver and kidney damage and death from circulatory failure. There is no antidote.