. >> on september 7th, 1978, this man, georgie markov a bulgarian work forget the bbc in london was waiting for a bus. somebody, the kgb was suspected, jabbed him in the back of the leg with an up presently larks four days later, markov was dead, killed by a ricin. >> how much ricin? >> just in the back of that tube. >> the chief investigator showed dan rather, the 60 minute correspondent. >> right here that tube contains a human lethal dose of sign need and, cyanide, and we have a lethal dose of ricin in there. >> that little speck, but now consider this. caster oil, the old-fashioned laxative, also comes from caster beans, and is harmless. >> the poison is in the dose. most things are toxic if they are eaten in large enough quantities or too much. >> this is the chemistry instrumentation room. >> tip patentor ads the poisonous plant research laboratory in logan, utah. >> for over a century, the u.s. department of agriculture has been sending teams to the renotest parts of the west to investigate plant poisonings. >> which cost ranchers more than $340 million a year in livestock losses. >>