Texas researchers will begin clinical trials of a candidate vaccine against the deadly toxin ricin (search), a biological agent that can only be tested in select labs.

University of Texas Southwestern Medical Center in Dallas received approval from the Food and Drug Administration to conduct the safety trial in humans.

According to the Centers for Disease Control and Prevention, as little as 500 micrograms of ricin — about what fits on the head of a pin — is enough to kill an adult. Lethal doses depend on how the poison is delivered — by powder, mist, pellet or dissolved in water. The poison can be made from waste left over from processing castor beans.

Because castor beans are easy to obtain and the poison remains potent despite exposure to temperature extremes, government officials worry that it could become a tool of terrorists. Unless treated quickly, there is no antidote. Symptoms arrive late and can be confused with other illnesses.

In October 2003, someone left a threatening letter containing ricin at a South Carolina postal facility. A second letter, addressed to the White House, was discovered by the Secret Service at a Washington offsite mail processing facility the next month.

The FBI is offering a reward of up to $100,000 for information leading to the arrest and conviction of those responsible for the October 2003 mailings of letters containing ricin. No arrests have been made and no suspects identified.

British authorities, meanwhile, are scheduled to begin trials for five men charged with conspiring to produce a chemical weapon. During that investigation, British police said they found traces of ricin.

The Texas researchers will test the safety of a genetically engineered protein vaccine, RiVax, developed by a team led by UT Southwestern's Dr. Ellen Vitetta.

People enrolled in the clinical trial will receive the vaccine. Then, their blood will be tested for their body's ability to generate protective antibodies. Mice, injected with the human antibodies against ricin, will be exposed to lethal doses of the poison to test the protective power of those human antibodies.

"As far as we can tell, the vaccine is completely safe and has no side effects," Dr. Vitetta, director of the Cancer Immunobiology Center at UT Southwestern, said in a statement.

The team developed the experimental RiVax vaccine for the deadly poison as an offshoot of its cancer research. The National Institutes of Health funded research to turn the discovery into a product that may soon help protect Americans against a bioterror attack.

In past trials, the vaccine protected mice against 10 lethal doses of ricin and had no side effects even when given at 100 times the dose needed to prompt an antibody response. Same for rabbits, which produced high numbers of protective antibodies against ricin after vaccination.

The early clinical trial is designed to confirm the vaccine is as safe in humans.