Former Ireland scrum-half Peter Stringer says he was "disgusted and angry" after appearing to be spat at during Bath's Amlin Challenge Cup defeat to Stade Francais.
Stade Francais' Jerome Fillol looks set to become the first professional player to be cited for spitting at an opponent.
Television cameras caught the French scrum-half spitting in the face of Stringer during the quarter-final at the Rec.
European Rugby Cup officials are to investigate the complaint that could see Fillol facing a lengthy ban.
Stringer said: "That's never happened to me before and there's no place for it in rugby.
"It's about as low as anyone can possibly go and I'm still a bit raw after being on the receiving end."
Bath have made a formal complaint over the incident during the first-half of Stade's 36-20 win.
Bath coach Toby Booth said: "Peter is very angry. I didn't see the incident but I would be disappointed if it's proved to be true because there's no place for it at all."
Stade Francais declined to comment, claiming they were unaware of the incident when they left the Rec on Saturday.
Under IRB regulations, the offence carries a four-week suspension for a low-end incident, rising to seven to mid-level, and at least 11 to a maximum of 52 weeks, for the worst offenders.
In December 2011, Uttoxeter wing Paul Millward was banned for a year for spitting at a female spectator during a Midlands 3 West (north) clash against Stourbridge Lions.
Fellow rugby stars rounded on Fillol (35).
Will Greenwood tweeted: "Fillol, good player but when you spit you cross a line. Disgrace."
Former Wales captain Gareth Thomas added: "He just spat in his face. No place for that. Get him off. Disgrace to the game."
And Harlequins wing Ugo Monye said: "Jerome Fillol is a disgrace. To spit at someone's face anywhere, let alone on the rugby pitch, is disgusting. Hope he gets banned for a long time."
Stade's victory leaves Leinster as the sole non-French team in the Amlin Cup semi-finals, alongside Perpignan and Biarritz.