With spring in the air, consumers are starting to open their wallets again. Retail sales rose 0.4 percent in March from February and a whopping 7.1 percent from March 2010.

That means people are beginning to use their credit cards again as well. Consumer credit began rising in January after two years of decline. Rising consumer credit means increased usage of credit cards, which fills the coffers of companies like American Express (AXP) and MasterCard (MA).

American Express

American Express, the biggest credit-card company by purchases, reported that its profit soared 48 percent in the fourth quarter from a year earlier, to $1.1 billion. Record spending by Amex customers and a decrease in reserves sparked the gain.

Other card companies make much of their money through interest payments on revolving credit and thus fight over balance transfers. But Amex makes most of its money from merchant fees, as its customers generally pay their balances in full each month. So all it has to worry about is enticing customers to spend more — usually a fairly simple task with American consumers.

Warren Buffet clearly believes in the stock. As of Dec. 31, his Berkshire Hathaway owned $6.5 billion in Amex shares, or 12.6 percent of the company.

MasterCard

The world’s second-biggest bank-card network saw its share price hit a 2½-year high recently. MasterCard’s profit climbed 41 percent in the fourth quarter to $415 million from $294 million a year earlier. The rebound in U.S. consumer spending and the increasing use of credit cards around the world were largely responsible.

MasterCard has gone on an acquisition spree to boost its presence in emerging markets.

In October, it bought DataCash Group, a U.K. credit-card processor, for $526 million. And in December, MasterCard announced it would purchase card-management assets from London-based Travelex Holdings for $470 million.

“We believe there are numerous long-term growth opportunities for MA, including international expansion, debit-card growth, mobile payments and prepaid cards,” writes Standard & Poor’s analyst Zaineb Bokhari, who has a four-star buy rating on the stock.