The interest rate charged by banks on loans made to broker-dealers, who use these loan proceeds to make margin loans to their clients. These broker's call loans are payable by the broker-dealer on call (i.e., immediately) upon request from the lending institution. The broker's call rate forms the basis on which margin loans are priced.
The broker's call rate may fluctuate on a daily basis, since it depends on a number of factors such as market interest rates, funds' supply and demand and economic conditions. It is published daily in publications such as The Wall Street Journal and Investor's Business Daily.