Scottish league chiefs have ordered investigations into fan behaviour at Ibrox and Tynecastle.
The Scottish Professional Football League has acted following two games between Christmas and New Year.
In a statement, the league said: “Following a review of the match delegate reports, the SPFL has instructed that formal investigations be undertaken in relation to the recent Rangers v Hibernian and Heart of Midlothian v Dundee United matches, where incidents of unacceptable conduct have been reported.”
Rangers fans were heard singing sectarian songs during their 4-2 win over Hibs on 28 December, including the “Billy Boys”.
The SPFL refused to expand on the issue under investigation at Tynecastle, but there were reports at the time of a disturbance in the Roseburn Stand during the game, next to the divide between both sets of fans.
An SPFL spokesman added: “The SPFL is committed to addressing unacceptable conduct where it occurs.”