After receiving protest letters from an attorney for mayoral candidate Tom Knox, two Philadelphia television stations yesterday decided not to run a TV spot criticizing Knox's business activities.

An anti-Knox group calling itself the Economic Justice Coalition for Truth had purchased about $40,000 worth of airtime to run the ad last night on stations WPVI (Channel 6) and WCAU (Channel 10).

Each station backed away after receiving a threatening letter from Knox's lawyer, Paul R. Rosen. The letter alleged that the ad was "false, misleading and illegal," and suggested that the anti-Knox group had tried to circumvent the city's campaign-finance laws.

The charges were denied by Alex Talmadge, a lawyer involved with the anti-Knox group.

A spokeswoman for WPVI, Caroline Welch, confirmed that Channel 6 had pulled the ad, but she said it was the result of an internal review that began before the station received Rosen's letter. She said management had "determined that there were claims made in this ad that needed to be substantiated."

As of last night, Welch said, the station was not fully satisfied with the documentation provided.

"That is the source of the station's decision not to run the ad," not Rosen's allegations, she said.

A spokeswoman for Channel 10, Eva Blackwell, said last night that she had been unable to reach any station executives familiar with the situation.

Talmadge said that the ad was accurate, based on articles in the Daily News and Inquirer, and that Rosen's charges were false.

"There is no evidence that the Economic Justice Coalition for Truth has circumvented any campaign finance laws whatsoever," Talmadge said in a phone interview last night.

He accused the TV stations of capitulating to pressure from Knox and Rosen, and called it a violation of the committee's rights of free speech.

"It is unconstitutional and we are considering our options," Talmadge said.

Using strong language, the 30-second TV spot calls attention to three items from Knox's business career:

* A $28,000 fee he received as a broker on insurance business from the Pennsylvania Convention Center - described in the ad as a "no-bid contract."

* A $70,000 fine levied by Maryland's insurance department against one of Knox's insurance companies, for various violations of Maryland insurance regulations. One violation involved instructions to policyholders to contact their primary physician before seeking treatment in a hospital emergency room.

* Knox's operating a bank that offered short-term loans with interest rates as high as 400 percent annually.

The ad was produced by Ken Smukler, a local political consultant who until last week was paid by Bob Brady's congressional- campaign committee. Brady is a candidate for mayor.

Smukler resigned from the post last Friday, after he acknowledged in an interview with the Daily News that he had also prepped Brady for events in the mayoral campaign and talked with Talmadge and others about setting up an independent political committee to run anti-Knox ads.

Talmadge said last night there had been no contact with the Brady campaign since the committee was created on April 4.

"I don't even know where Brady's office is," Talmadge said.

Smukler said he began working on the ad last Friday, "actually the minute I resigned . . . ."

He said he completed the ad early this week and posted it on YouTube on the Internet, publicly urging Talmadge's group to put it on commercial TV. *