Ricin is a highly potent toxin produced in the seeds of the castor oil plant, Ricinus communis. It is classified as an extremely hazardous substance in the United States and is subject to strict reporting requirements by facilities that produce, store, or use it in significant quantities. Ricin is a toxalbumin, a biological toxin whose mechanism of action is inhibition of protein synthesis in eukaryotic cells, resulting in cell death. Ricin toxin can be extracted from castor beans, purified and treated to form a pellet, a white powder, or dissolved in water or weak acid to be released as a liquid. Ricin is stable under ambient conditions and particles of <10 µm have been used for animal studies characterizing inhalational exposure. Ricin particles can remain suspended in undisturbed air for several hours, and re-suspension of settled ricin from disturbed surfaces may also occur.

Ricin is easily extracted from the castor bean and refined into an oil or powder form, making it possible to target individuals or small groups of people. It is easily weaponized when used as a contaminant for food, water, or aerosol sprays. Exposure to ricin can cause severe and painful symptoms, and a dose no larger than a few grains of salt can kill a healthy adult. There is no vaccine, no antidote, and no cure for ricin poisoning. Ricin is generally viewed as an assassination weapon, given that it is tasteless, odorless, and difficult to detect both pre- and post-mortem. Its utility as a food or water contaminant is also limited due to a dilutional effect that would require large amounts of ricin for any appreciable effect.