Enforcement of the ban, which was adopted in 1992, has been spotty, The Detroit News reported Saturday. Notices were posted this year as protests by union members began building.
Steve Benkovsky, head of the Legislative Council Facilities Agency for the past three years, said he has been stricter about enforcing the rule than most of his predecessors.
Ray Holman, an official with United Auto Workers Local 6000, said he was not allowed to enter the building this week holding a cardboard sign that said "Collective bargaining is a human right."
Michael Steinberg, legal director of the ACLU's Detroit chapter, said he notified officials this week that he believes the sign ban is unconstitutional.
"There is obviously no reason why people can't carry small signs to express their views on the most important issues of the day," he said.