Championship club Millwall said they will issue a lifetime ban to any supporters found guilty of making homophobic chants during their FA Cup third-round defeat by Crystal Palace.
There were audible homophobic chants directed at on-loan Chelsea midfielder Conor Gallagher.
A bottle was thrown from the stands at Michael Olise.
A section of the home crowd also booed Palace players taking the knee before kick-off.
Millwall, who lost 2-1 at The Den, have opened an investigation into the chanting.
Away fans also let off a smoke bomb in the stand and threw a flare onto the pitch after Jean-Philippe Mateta's 58th-minute winner for Palace.