In a monetary system in which risk-free and risky money coexist, Gresham's law predicts that people will prefer to hoard risk-free money as a store of value and spend risky money as a medium of exchange. Establishing a payment system on the basis of risk-free money, such as a retail CBDC, while maintaining the fractional reserve banking system in place poses numerous challenges. In a laboratory experiment, we demonstrate that when the holding of risk-free money is unrestricted, people hold and pay with it extensively. However, when the ability to hold risk-free money is limited by a ceiling or an unattractive interest rate, people tend to hoard risk-free money and use risky money for payments.