The organisers of the Champions Cup have said they are in contact with Ulster and Racing 92 after Simon Zebo tweeted about "comments directed my way from the crowd".
Ulster beat Racing 26-22 on Saturday with Zebo scoring one of his side's four tries in the game.
After the match, Zebo took to Twitter to complain about some of the things he heard from the crowd.
"A tough place to play but great effort from the boys, 2 important points on the road.
"Also I hope my ears deceived me with some comments directed my way from the crowd #NotOn Django wins in the end."
The Django statement appears to be a reference to Django Unchained, a 2012 film about slavery and racism in the US in 1858.
European Professional Club Rugby (EPRC) released a statement to RTÉ Sport saying: "EPCR has not received a formal complaint from Racing 92 regarding any alleged case of verbal abuse of one of the club’s players during yesterday’s Heineken Champions Cup, Pool 4 match at the Kingspan Stadium.
"EPCR is in contact with both Ulster Rugby and Racing 92, and will be making no further comment at this point."
Ulster released a statement saying: "Ulster Rugby condemns all forms of abuse and will work with the relevant parties to robustly investigate any complaints received."
Responding to Zebo's tweet, former Ulster player Neil Best said:
"Racism has no place in sport and no place in any other area of life. But vague hints/suggestions something may have been said does a great disservice to the many @ulsterrugby fans who were hostile not based on any player’s race/ethnicity. Complaint/Investigation/Outcome/Process."
Zebo was booed during the clash, believed to be after the full-back made a gloating gesture toward Ulster's Michael Lowry when the side's met in October.
The full-back pointed his finger at the Ulster player in a taunting manner whilst in the process of crossing the line, but he later apologised for the action.
On Twitter after that game, Zebo wrote: "Really sorry to Michael Lowry.
"Got too excited and I don’t do stuff like that as long as I’ve played. I wouldn’t like my kids doing that and I’m disappointed in myself.
"Made a mistake, not in the spirit of the game..keep up the great work. Sorry again."
