Open market operations are done through the sale and purchase of United States Treasury security.

http://en.wikipedia.org/wiki/Federal_Reserve_System#cite_note-91

Open market operations allow the Federal Reserve to increase or decrease the amount of money in the banking system as necessary to balance the Federal Reserve's dual mandates. Open market operations are done through the sale and purchase of United States Treasury security, sometimes called "Treasury bills" or more informally "T-bills" or "Treasuries".