Blues captain Keven Mealamu was hit in the head by a bottle following his side's 29-23 victory over the Bulls at Loftus Versfeld in Pretoria this morning.
Mealamu was not injured in the incident, which happened at the end of the match after a spectator jumped the pitch-side fence and confronted him.
"He's obviously a bit annoyed by it. He's not happy but he's okay now,'' Blues coach Pat Lam said.
"He's in a bit of trouble,'' Lam said of the offender, a thin man of advancing years dressed in a Bulls jersey. "It's in the hands of the police. They have the footage and everything and so we'll just go through the process.''
Blues halfback Piri Weepu tweeted afterwards: "To the clown that jumped the fence and threw a bottle at Kevin's (sic) head!! Your (sic) a loser!!''
The man was shepherded off the pitch by Bulls wing Bjorn Basson, who minutes earlier been injured by an illegal Rene Ranger tackle in the act of scoring a try in the corner.
Ranger was shown a yellow and white card by referee Steve Walsh for not using his arms in the tackle, and was later cited for dangerous play. If found guilty he faces a suspension.
The victory was the Blues' first in 2012 after losses to the Crusaders and Chiefs. Rookie first-five Gareth Anscombe scored all of the Blues points with two tries, two conversions and five penalties.