The proposal reflects mounting concern about exposure to mercury in various forms. And it comes against a backdrop of charges that mercury, a neurotoxin, has contributed to a surge in autism and other neurological ailments in children.

The vaccine industry and many public health officials say there is no proof of any harm to children from the small amounts of mercury in vaccines.

Even so, manufacturers in recent years have voluntarily eliminated thimerosal or reduced it to trace levels in routine pediatric vaccines. The flu vaccine made by Aventis — the lone supplier of flu vaccine for children younger than 2 — is the only major exception.

Aventis offers its vaccine with or without the preservative. Under terms of the bill, it would have two years to convert to entirely thimerosal-free flu vaccine.

After months of taking no position on the Pavley bill, Aventis is raising the specter of shortages that could leave the state vulnerable to a mass outbreak of flu. In a statement to The Times, Aventis warned that the bill “would curtail the access of Californians to needed vaccine and undermine public confidence in one of healthcare’s most effective prevention tools.”

Earlier this year, however, an Aventis spokesman said the company should be able to produce enough thimerosal-free vaccine to fill all orders, given sufficient notice. Aventis representatives did not return calls seeking clarification.

Rick Rollens, a Granite Bay resident with an autistic son and an activist in issues of vaccine safety, described Aventis’ stand as indefensible and “a bully tactic by a large pharmaceutical company.”

Aventis previously stayed on the sidelines, leaving the state chapters of the American Academy of Pediatrics and American Academy of Family Physicians to oppose the bill.

But both groups dropped their opposition when Pavley agreed to amendments extending the compliance date for flu vaccine to July 2006 and providing for suspension of the ban in a public health emergency.

Aventis and the health officers declared their opposition in letters to Pavley’s office in the last week.

The conference of health officers’ letter echoed Aventis’ warning that the bill could have dire consequences in the event of an influenza pandemic.

In response to inquiries by The Times, Morrow said he discovered that Aventis sought the group’s help on the bill — something he said he didn’t know at the time he signed the letter.

“I’m very disappointed,” he said. “It feels to me very disingenuous.”

Nonetheless, Morrow said, his group has taken the right “position regardless of how it came to us.”