WASHINGTON - Families claiming that a mercury-based preservative in vaccines triggers autism will challenge mainstream medicine Monday as they take their case to a federal court.

They seek vindication and financial redress from a government fund that helps people injured by shots.

Two 10-year-old boys from Portland, Ore., will serve as test cases that determine whether the children and their families in similar situations should be compensated. Attorneys for the boys will attempt to show the boys were happy, healthy and developing normally. But, after being exposed to vaccines with thimerosal, they began to regress and show symptoms of autism.

Thimerosal has been removed in recent years from standard childhood vaccines, except flu vaccines that are not packaged in single-doses. The CDC says single-dose flu shots currently are available only in limited quantities. In 2004, a committee with the Institute of Medicine concluded there was no credible evidence that vaccines containing thimerosal caused autism.

Overall, nearly 4,900 families have filed claims with the U.S. Court of Claims alleging that vaccines caused autism and other neurological problems in their children. Lawyers for the families will present three different theories of how vaccines caused autism.

The Office of Special Masters of the claims court has instructed the plaintiffs to designate three test cases for each of the three theories — nine cases in all — and has assigned three special masters to handle the cases. Three cases in the first category were heard last year, but no decisions have been reached.

The two cases beginning Monday are among the three that focus on the second theory of causation: that thimerosal-containing vaccines alone cause autism. The plaintiff in the third case originally scheduled for hearing this month has withdrawn and lawyers and court officials are working to agree on substitute case.

Hearings in the test cases for the third theory of causation are scheduled in mid-September.

Lawyers for the petitioning families in the cases being heard this month say they will present evidence that injections with thimerosal deposit a form of mercury in the brain. That mercury excites certain brain cells that stay chronically activated trying to get rid of the intrusion.

"In some kids, there's enough of it that it sets off this chronic neuroinflammatory pattern that can lead to regressive autism," said attorney Mike Williams.

In the end, the families' attorneys hope to convince the special master hearing their case that thimerosal belongs on the list of causes for the inflammation that leads to regressive autism.

To win, the attorneys for the two boys, William Mead and Jordan King, will have to show that it"s more likely than not that the vaccine actually caused the injury.

Many members of the medical community are skeptical of the families' claims. They worry that the claims about the dangers of vaccines could cause some people to forgo vaccines that prevent illness.

"I think that what's so endearing to me about the anti-vaccine people is they're perfectly willing to go from one hypothesis to the next without a backward glance," said Dr. Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia.

Autism is a developmental disability that typically appears during the first three years of life and affects a person's ability to communicate and interact with others. Dr. Andrew Gerber, a psychiatrist, said that medical experts don't have a comprehensive understanding of what causes autism, but they do know there is a strong hereditary component.

Toxins from the environment could play a role, but currently, data does not support that they do, Gerber said.

Arguments are scheduled to go on throughout the month. A final decision could take several more months. Claims that are successful would result in compensation taking into account lost earnings after age 18 and up to $250,000 for pain and suffering.

The families or the federal government can also appeal the decision of the special master to the Court of Federal Claims or to a federal appeals court.

The court Web site says more than 12,500 claims have been filed since creation of the program in 1987, including more than 5,300 autism cases, and that more than $1.7 billion has been paid in claims. It says there is now more than $2.7 billion a trust fund supported by an excise tax on each dose of vaccine covered by the program.