Experts say it is more effective for use against individuals than as a weapon of mass destruction.
Ricin was used in the 1978 assassination of Bulgarian dissident Georgi Markov. The author, who had defected nine years earlier, was jabbed by the tip of an umbrella while awaiting a bus in London and died four days later.
A ricin scare hit the Capitol in 2004, when tests identified it in a letter in a Senate mail room that served then-Majority Leader Bill Frist's office. The discovery led 16 employees to undergo decontamination; none was sickened, Frist said.