31 C.F.R. § 309.4
The income derived from Treasury bills, whether interest or gain from the sale or other disposition of the bills, shall not have any exemption, as such, and loss from the sale or other disposition of Treasury bills shall not have any special treatment, as such, under the Internal Revenue Code, or laws amendatory or supplementary thereto. The bills shall be subject to estate, inheritance, gift or other excise taxes, whether Federal or State, but shall be exempt from all taxation now or hereafter imposed on the principal or interest thereof by any State, or any of the possessions of the United States, or by any local taxing authority. For purposes of taxation the amount of discount at which Treasury bills are originally sold by the United States shall be considered to be interest.
31 C.F.R. § 309.4