Compliance With Procedural Rules
The second limitation on Congress' subpoena power is that,
for a subpoena to be valid, it must be authorized and issued in
compliance with the procedural rules applicable to the commit-
tee or
Exxon Corp. v. FTC:
To issue a valid subpoena, however, a committee or subcom-
mittee must conform strictly to the [rules] establishing its
investigatory powers, and only those parties expressly autho-
rized to sign
where the [rules] granting subpoena power to a committee
stated that subpoena would be issued only by the whole com-
mittee, not even the chairman himself could individually issue such a document...
Rather, be-
cause of jurisprudential and separation of power consider-
ations, courts generally will act to protect against invalid sub-
poenas only after Congress has brought contempt proceedings
against a party
When this occurs, an invalid subpoena can be asserted as a defense to a prosecution for contempt of Congress.
the third requirement, that the testimony or materi-
als sought should be pertinent to the inquiry, has served as a
basis for invalidating Congressional subpoena... Which was already released