27 CFR § 72.25 - Deposit of collateral.
(a) Bonds or notes of the United States, or other obligations which are unconditionally guaranteed as to both interest and principal by the United States, may be pledged and deposited by claimants as collateral security in lieu of corporate sureties in accordance with the provisions of Treasury Department Circular No. 154 (31 CFR Part 225 - Acceptance of Bonds, Notes or Other Obligations Issued or Guaranteed by the United States as Security in Lieu of Surety or Sureties on Penal Bonds). Alternatively, cash, postal money orders, and certified or cashiers' or treasurers' checks may be furnished by claimants as collateral security in lieu of corporate sureties.
(b) Treasury Department Circular No. 154 is periodically revised and contains the provisions of 31 CFR Part 225 and the forms prescribed in 31 CFR Part 225. Copies of the circular may be obtained from the Audit Staff, Bureau of Government Financial Operations, Department of the Treasury, Washington, DC 20226.