29 C.F.R. § 1626.21
In any action or proceeding based on any act or omission on or after the date of the enactment of this Act, no employer shall be subject to any liability or punishment * * * if he pleads and proves that the act or omission complained of was in good faith in conformity with and in reliance on any written administrative regulations, order, ruling, approval or interpretation * * * or any administrative practice or enforcement policy of [the Commission].
The Commission has determined that only (1) a written document, entitled "opinion letter," signed by the Legal Counsel on behalf of and as approved by the Commission, or (2) a written document issued in the conduct of litigation, entitled "opinion letter," signed by the General Counsel on behalf of and as approved by the Commission, or (3) matter published and specifically designated as such in the FEDERAL REGISTER, may be relied upon by any employer as a "written regulation, order, ruling, approval or interpretation" or "evidence of any administrative practice or enforcement policy" of the Commission "with respect to the class of employers to which he belongs," within the meaning of the statutory provisions quoted above.
29 C.F.R. §1626.21