Merchants, who have long chafed at American Express Co.'s fees and some of its policies, have not sued it as often as they have pursued antitrust claims against MasterCard Inc., Visa Inc., and their issuing banks.

But this week three large drugstore companies filed lawsuits alleging that provisions in agreements with Amex violate antitrust laws by restraining them from steering customers to competing payment cards.

CVS Caremark Corp. of Woonsocket, R.I., Walgreen Co. of Deerfield, Ill., and Rite Aid Corp. of Camp Hill, Pa., filed the complaints in the U.S. District Court for the Eastern District of New York — the same court hearing a consolidated merchant suit against Visa and MasterCard.

In its complaint, CVS said contracts with Amex prohibit merchants from "doing anything that would lead … a retail purchaser to use a lower-priced payment card," as well as "'criticizing' the Amex card or services" or promoting the use of competing products, among other things.

Refusing to accept Amex cards is "theoretically" an option for merchants but is "not economically feasible or realistic," the suit says. "If CVS or the other merchants stopped accepting Amex cards, they would have lost many of those customers to competitive retailers."

The suits by Walgreen and Rite Aid make similar claims.

An Amex spokeswoman said the company does not see any merit in the suits and will "vigorously defend our position."

Attempts to speak with attorneys for the retailers by press time were unsuccessful.

The issues that the drugstore companies raised in their suits "have been out there for a long time," said Thomas Brown, a partner in the San Francisco office of O'Melveny & Myers LLP. "There has been historically merchant discontent with this aspect of the American Express card agreement."

Adam Levitin, an associate professor at Georgetown University Law Center, said that retailer litigants have concentrated their energies on Visa and MasterCard because they are larger, so it is easier to make the case that they have market power, and because "you can't have a conspiracy of one."

Mr. Brown said that "as a legal matter, it may have been harder in the past to sue American Express for issues that relate to the cost of payment acceptance" than to sue Visa or MasterCard. The demutualizations of MasterCard in 2006 and Visa this year "should make it more difficult for plaintiffs to sue" them, "not less difficult to sue American Express."

"Amex … for a long time successfully presented itself as different" from Visa and MasterCard in terms of vulnerability to litigation "for reasons having to do with its unitary corporate structure, because it was the only issuer and the only acquirer," he said. "As these complaints illustrate, I think maybe that's no longer the case."

In a February filing with the Securities and Exchange Commission, Amex described a suit filed against it in April 2006 that alleged the use of illegal "anti-steering" rules. Amex said that litigation had been stayed pending the appeal of an arbitration ruling in another group of antitrust cases.

(In 2004, Amex sued Visa and MasterCard over exclusionary rules that until that year prevented their member banks from issuing cards on other networks. Visa settled the case for up to $2.25 billion last year; the case against MasterCard is still pending.)

Mr. Brown said he found the cases brought by the drugstore companies unconvincing. The plaintiffs are relying on "this notion that there's a relevant antitrust market that can be defined as American Express payment card services, and that should be an issue that American Express easily dispenses with," he said. "There's lots and lots of authority for the proposition that we don't define antitrust markets at the level of particular branded products."

David Balto, an antitrust lawyer in Washington who served as the policy director of the Bureau of Competition of the Federal Trade Commission from 1995 to 2001, said there is "a lot of evidence" that Amex "could very well have market power," regardless of its market share of about 25%.

"Where there's direct evidence of market power, such as the ability to significantly increase prices, market definition and market share is a secondary and less important exercise," Mr. Balto said. Any kind of card company "would have a difficult time explaining why an anti-steering provision is justified," he said. "It's a way of limiting merchants' options and ultimately consumer choice."

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