Ricin toxin, found in the bean of the castor plant, is one of the most toxic and easily produced plant toxin. It is composed of two polypeptide chains linked by a disulfide bond. The toxin irreversibly blocks protein synthesis. Oral intoxication is the most frequent mode of exposure, but direct contact and inhalation have also been described. A single intramuscular exposure was reported, related to an assassination event in 1978. The clinical signs of intoxication begin hours after the exposure, and include gastrointestinal signs, dyspnea, fever, vasculatory collapse, neurological signs, liver necrosis and renal failure. Treatment is mainly supportive. A vaccine has been developed and found effective in animal studies. Ricin may be used as a biological weapon due to its heat stability and worldwide availability. A more friendly use, which has been tested in clinical trials, is the incorporation of ricin-immunotoxin for the treatment of cancer patients.