You can purchase and hold eligible Treasury securities in your account. Eligible securities are Series EE and Series I savings bonds, zero-percent certificates of indebtedness, and marketable Treasury securities that are available for purchase through the TreasuryDirect Web site. In addition, you can hold converted savings bonds and eligible marketable Treasury securities that have been transferred from the Legacy Treasury Direct system or the commercial book-entry system.
31 C.F.R. §363.36