Mastercard is being investigated by the European Commission over fees charged for card transactions made by people visiting Europe.
The commission says some of the firm's inter-bank fees and related practices may be anti-competitive, the BBC reports.
It is already investigating rival Visa over similar practices.
Mastercard, which says it will fully co-operate with regulators, could be fined up to $US740 million, or a tenth of its 2012 revenue, if found guilty.
The investigation will examine payments made by people from outside the European Economic Authority who use their credit and debit cards when inside the area.