Ricin Ricin is poisonous if inhaled, injected, or ingested, acting as a toxin by the inhibition of protein synthesis. While there is no known antidote, the US military has developed a vaccine. The best-known documented use of ricin as an agent of biological warfare was by the Soviet Union’s KGB during the Cold War. In 1978, the Bulgarian dissident Georgi Markov was assassinated by Bulgarian secret police who surreptitiously ‘shot’ him on a London street with a modified umbrella using compressed gas to fire a tiny pellet contaminated with ricin into his leg