Rush Limbaugh told his radio listeners Monday that actor Michael J. Fox, who has Parkinson's disease, was ''exaggerating the effects of the disease" in political ads supporting stem cell research.

"He's moving all around and shaking, and it's purely an act. ... This is really shameless of Michael J. Fox. Either he didn't take his medication or he's acting," Mr. Limbaugh said.

Later Monday, still on the air, Mr. Limbaugh apologized, but then he leveled a different criticism at the actor, who is appearing in commercials for several candidates supporting stem cell research.

"Michael J. Fox is allowing his illness to be exploited and in the process is shilling for a Democratic politician," Mr. Limbaugh said.

Reaction to his statements from Parkinson's experts and Mr. Fox's supporters was swift and angry.

"It's a shameless statement," John Rogers said Tuesday. Mr. Rogers, the actor's political adviser, who also serves on the board of the Parkinson's Action Network, added: "It's insulting. It's appallingly sad, at best."

"Anyone who knows the disease well would regard his movement as classic severe Parkinson's disease," said Elaine Richman, a neuroscientist. "Any other interpretation is misinformed."

Mr. Limbaugh, whose syndicated radio program has a weekly audience of about 10 million, was reacting to Mr. Fox's appearance in a spot for Missouri Democrat Claire McCaskill, who is running against Republican Sen. James Talent.

Mr. Fox also appears in an ad for Maryland Democrat Ben Cardin, who is running against Republican Michael Steele for a Senate seat. As he speaks in the ad, Mr. Fox's torso weaves and writhes. His head bobs from side to side, almost leaving the video frame.

"Stem cell research offers hope to millions of Americans with diseases like diabetes, Alzheimer's and Parkinson's," Mr. Fox says in an ad for Mr. Cardin.

Mr. Fox was campaigning outside Chicago on Tuesday for Tammy Duckworth, a congressional candidate who lost her legs while serving in Iraq. Mr. Rogers, who was with the actor, said Mr. Fox had no public comment on Mr. Limbaugh's statements.

"It's a never-ending chain of hope that would be set off by the proper use of this wonderful material," Mr. Fox said of stem cells during his appearance with Ms. Duckworth. "Today is a good day," he joked. "But it could go to hell."

The Washington Post and the Chicago Tribune contributed to this report.