Professional Footballers' Association chief executive Gordon Taylor has confirmed the union has been contacted by Sunderland players about their manager Paolo Di Canio and that it is investigating the recent fines issued by the Italian.
Di Canio, appointed as successor to the sacked Martin O'Neill in March, has been speaking publicly about what he sees as a lack of discipline among the players at the Wearside club, seven of whom he fined last week.
Defenders Phil Bardsley and Matt Kilgallon were dropped from the matchday squad for Sunday's Barclays Premier League clash at Tottenham after they were pictured celebrating on a night out in a casino earlier in the week, with the former lying on the floor covered in Â£50 notes.
In his post-match press conference at the weekend, Di Canio said: "I thought that at (his previous club) Swindon (there were) arrogant, ignorant footballers because they've not had many chances to stay at the top level. I have to tell you unfortunately I found a worse environment in terms of discipline in this club."
That was part of a lengthy rant about his players from Di Canio, who stressed the need for the matter to be addressed and also claimed the PFA "excuse people that behave like Phil Bardsley...(even though) the PFA don't pay them, it's a private club that pays them."
Taylor, without being drawn on specific names or details of the fines, confirmed that members of the Sunderland squad had been in touch with the PFA regarding former West Ham midfielder Di Canio, and emphasised that the union were examining the situation closely.
Taylor told Press Association Sport: "We know Paolo as a player and a manager, and he is a bit volatile.
"But he knows, from when he was a player with the PFA, what we are about. We represented him then, the same as we also represent the players (now).
"One point he made was about Sunderland being able to do what they want - well they can't. There is a disciplinary procedure that is agreed.
"We obviously agree with discipline, but that has to be in line with the Football Association and the Premier League. From that point of view, there is a due process."