American Express will buy the rest of First Data.

The company said it plans to acquire complete ownership of First Data Resources by purchasing the 25% of the stock that it does not yet own for about $225 million. First Data processes credit card transactions and has interests in electronic marketing and other computer services and already is 75% owned by an American Express subsidiary. American Express said details of the transaction, which it valued at $36 a share, remain to be negotiated with First Data.