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 the test cases that determine whether the children and their families 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 case beginning Monday focuses on the second of those theories: that thimerosal-containing vaccines alone cause autism. Lawyers for the petitioning families said 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 a special master of the U.S. Court of Claims 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.