Ricin. Molecular model of the toxic protein ricin with ligands attached. It comprises two entwined amino acid chains, termed A and B. 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), making it easy to be used as a toxin by bioterrorists. 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.