American Express won a contract with Citigroup, the world’s biggest credit card company. A month ago, the first American Express card was issued by MBNA, the third-biggest US credit card company, wiping away the restrictive ruled that had kept the American Express out of the banking system.

"This demonstrates how leading banks that could not partner with us because of the illegal barriers that were in place see a value in a partnership and a relationship with American Express," said Kenneth I. Chenault, the chairman and chief executive of American Express.

The contract came in the wake of the decision by the Supreme Court to reject an appeal from MasterCard Inc. and Visa U.S.A. against a federal court ruling to dismantle their barrier system prohibiting their member banks from partnering with other credit card brands like American Express and Discover. Now that the barriers to such partnerships have been removed, more banks can be expected to try these cards.

MasterCard said in a statement, "Given the competitive nature of the payments industry, this kind of experimentation is to be expected. The real test will be how much traction these programs get over the long term."

"The associations said years ago, if the bylaws came down, banks in fact would not want to work with American Express," Chenault said. "Obviously, we thought that was ludicrous then, and I think this clearly demonstrates that banks strongly see the value of working with American Express and issuing on our network."