Thomas Brinker loves to sing and play with string. He watches ABC News anchor Peter Jennings on television every night and shouts: "Tickle Peter Jennings." He's 8 now, but his attention span is short and his temper flares easily.

Thomas has autism, a condition his parents believe was caused by a simple childhood immunization. "We're waiting for his first normal moment," said his mother, Donna Brinker of Glen Mills, Pa.

It was Donna Brinker's temper that flared when she learned that Congress had quietly restricted her right to sue Eli Lilly and Co. and other manufacturers of Thimerosal, the mercury-based vaccine preservative she believes caused her son's condition. The change came in two paragraphs tacked onto the massive Homeland Security Act just days before Congress approved the legislation in November.

The Brinkers are among 800 families in more than a dozen states that have filed similar cases seeking compensation for the costs of their children's autism. Under the new law, signed by President Bush Nov. 25, the parents are required to file claims with a special administrative court under the National Vaccine Injury Compensation Program before they can take their cases to civil court.

The changes could sharply reduce parents' chances of prevailing in civil courts, where damage awards normally could be much higher than those in the "vaccine court." The federal program covers claims for medical and education expenses, but damages for pain, suffering and death are limited to $250,000. Lawyers for the plaintiffs say their awards would likely be higher if they could first take their cases to state courts, where civil juries are known to award millions of dollars in medical injury cases.

Meanwhile, the Department of Justice has filed a request to restrict the use of information gathered in vaccine court proceedings in subsequent civil court cases, another potential obstacle for the plaintiffs.

"I felt betrayed," Brinker said of the new legislation. "I believe in protecting our homeland, but it petrifies me to think that our nation would protect any industry at the expense of our children."

Penny Starr-Ashton, of Drexel Hill, Pa., whose autistic 6-year-old daughter, Maddie, is another plaintiff in a class-action lawsuit filed in Pennsylvania in July, said it is particularly painful to have the provision wrapped in the flag.

"Who doesn't want a safer country?" she asked. "But who's going to protect me? Who's going to protect my child?"

The National Institute of Child Health and Human Development estimates that between 1 in 500 and 1 in 1,000 children is diagnosed with autism in the United States each year. Initial studies in the 1960s found four to five cases of autism in every 10,000 people, although the institute cautions that some of the increase could be due to changes in reporting and diagnosing the disease.

A study by the University of California at Davis found that a third of California parents of autistic children diagnosed in the mid-1990s blame vaccines for their children's illnesses.

Congress created the National Vaccine Injury Compensation Program in 1986 to address growing concerns about vaccine safety. Claims are filed with the Department of Health and Human Services through the U.S. Court of Federal Claims. The program has paid out 1,775 claims totaling $1.4 billion and is funded by a 75-cent surcharge on every child vaccination.

Brinker said parents of children with signs of mercury poisoning can spend up to $20,000 a year out of pocket. Thomas is undergoing chelation therapy to draw metals out of his body and is on a strict diet. His parents take him to a specialist in Louisiana for treatment, and his mother travels to Mexico to get drugs that are not approved in the United States.

Beyond today's expenses, Brinker worries about supporting Thomas in the long term. "The mercury preservative has deprived Thomas of having a normal life," she said. "That our nation would protect such a killer is beyond comprehension."

Aside from potentially lower awards, Thomas Brinker and Maddie Ashton will have another problem in vaccine court, said their lawyer, Tobi Millrood. Like many children, they were diagnosed with autism more than three years after their vaccinations, beyond the time permitted to file under the program's rules.

Some states, including Oregon, Florida, Louisiana, Illinois and California, had ruled that they had jurisdiction over Thimerosal cases, said John Kim, a Houston lawyer who argued against the government's request to close vaccine court records. "Now I guess this new provision in the Homeland Security Act trumps that," Kim said.

Meanwhile, all Thimerosal cases have been put on hold at vaccine court while the court grapples with the scientific debate over the possible causes of autism. The Office of the Special Master, which oversees procedural issues at vaccine court, expects 3,000 to 5,000 filings.

Parents outraged about the last-minute change point to Eli Lilly, the Indianapolis drug maker, as its biggest beneficiary. Lilly invented Thimerosal and manufactured it until the 1980s. The preservative is 50 percent mercury by weight, and had been used in vaccines since the 1930s. Lilly is a defendant in 200 Thimerosal-related lawsuits.

"It's turned into being about money," Brinker said. "Parents with kids with autism don't have the money to give to congressmen. It turns out whoever has the most money wins."

The provision in the Homeland Security bill was originally written by Sen. Bill Frist (R-Tenn.), a physician, as part of broader legislation aimed at helping drug companies produce vaccines after post-Sept. 11, 2001, concerns about smallpox and anthrax. The number of U.S. vaccine manufacturers has dropped to four, with companies complaining of low profit margins, manufacturing problems and fear of liability for injury.

Edward G. Sagebiel, a spokesman for Lilly, said his company had no role in pushing the last-minute legislative changes. "We express sympathy for the parents and the children who have suffered adverse reactions," he said. "However, the lawsuits that have been filed against Lilly and other manufacturers are not supported by science."

The House Government Reform Committee has scheduled a hearing on vaccine safety for Tuesday.

In 1999, the Food and Drug Administration conducted a review of Thimerosal and found no evidence of harm beyond limited cases of hypersensitivity to the vaccine. But the same year, the Academy of Pediatrics and the U.S. Public Health Service recommended that Thimerosal be removed from vaccines, partly out of fear that parents would stop immunizing their children and create a bigger public health problem.

In October 2001, the Institute of Medicine, a branch of the National Academy of Sciences, said there was no evidence that Thimerosal caused autism, but it did say the theory was "biologically plausible."

Most recently, on Nov. 30, the British medical journal the Lancet published a study showing that infants who received vaccines containing Thimerosal had levels of mercury in their blood that are within federal limits.

Starr-Ashton remains unconvinced. "I don't believe anything that is 50 percent mercury by weight is safe," she said. She noted reports of health damage caused by mercury in fish, thermometers and dental fillings. "I'm not that dumb."

The debate over science has become a furor over the democratic process in the tight-knit community of parents of children with autism that is linked by the Internet and community support groups.

"Nobody is owning up to it," Brinker said. "It is so underhanded. I just can't believe our government would do this. We're not going to back down on this issue. We will not be silent."

Starr-Ashton said she is not against vaccines, especially because she taught in a school for the deaf for many years: "I saw first-hand the damage done by rubella."

But now she does not know who to trust. "Here I was, a dutiful parent taking my child to do what the government and the Academy of Pediatrics said I should do to protect my child against disease," Starr-Ashton said. "Something went terribly wrong. I need answers."

Tom and Donna Brinker, of Glen Mills, Pa., with their son, Thomas, 8. The boy is a plaintiff in a class-action suit filed in Philadelphia alleging makers of the preservative Thimerosal caused him to develop autism.Penny Starr-Ashton of Drexel Hill, Pa., holds her 6-year-old daughter Maddie. The girl is also suing the makers of Thimerosal because of her autism.