In the biggest IPO of the year, MasterCard Inc. shares surged in their stock market debut Thursday even though the world's No. 2 credit-card brand originally priced below expectations.

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MasterCard shares rose $3.94, or 10 percent, to $42.40 shortly after the stock opened for the first time in morning trading on the New York Stock Exchange. The initial public offering raised $2.39 billion.

The long-awaited IPO will go down as one of the market's biggest in the past two years, easily eclipsing the $1.7 billion raised when Google Inc. (GOOG) went public in 2004.

Still, the size of the initial public offering came up short of the company's original expectations as MasterCard fell victim to a volatile market and concerns over its mounting legal problems. The shares priced at $39 apiece late Wednesday — below its original forecast for a range of $40 to $43 a share.

Weighing on the price were several factors, including recent erratic market conditions that led to a lackluster IPO from Vonage Holdings Corp. (VG) on Wednesday. The country's leading Internet phone provider's stock fell 13 percent below its initial offer price.

But what might have been the main culprit was investor concern over MasterCard's legal and regulatory problems. Architects of the IPO designed it as a defensive move to shield the company from a legal assault by retailers who feel fees are too high and continued regulatory concern over antitrust issues.

Some 61.52 million shares — representing a 46 percent stake in the company — began trading Thursday under the symbol MA.

The deal valued Purchase, N.Y.-based MasterCard — which is owned by its 1,400 member banks — as an almost $6 billion company.

Proceeds from the deal will mostly be used to redeem Class B shares, allowing the banks that make up MasterCard's association to begin unwinding their stakes. MasterCard will also use about $650 million raised in the public flotation to fund a war chest to protect itself from mounting legal troubles.

MasterCard and larger rival Visa International face ongoing legal battles over what are known as interchange fees, which retailers pay the associations to process credit and debit card transactions. Merchant groups have already filed a class-action suit alleging unlawful price fixing of fees that hurt both merchants and consumers.

In addition, both MasterCard and Visa have also been sued by American Express Co. (AXP) and Discover Financial Services LLC, the credit card division of Morgan Stanley, for anticompetitive practices that blocked member banks from issuing cards on their rival networks.

Visa, which is also owned by member banks, has said it doesn't plan to go public

MasterCard is expected to use its new status as a public company to expand globally, and move into higher growth businesses within the payment industry.

The company recently posted a $126.7 million profit for the quarter ending March 31, up almost 36 percent from the year-earlier period, according to a filing with the Securities and Exchange Commission. MasterCard also reported a 12 percent increase in revenue, to $738.5 million during the quarter.

Citigroup Global Markets and Goldman Sachs & Co. are the lead underwriters on the stock sale.

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