Knight has since revealed his co-star did use the 'f' word on the set, and was shocked and angry he used it again backstage at the Globes.
And now GLAAD officials have taken aim at Washington.
President Neil Giuliano says, "When Isaiah Washington uses this kind of anti-gay slur--whether on set or in front of the press--it does more than create a hostile environment for his castmates and the crew of Grey's Anatomy.
"It also feeds a climate of hatred and intolerance that contributes to putting our community in harm's way. Washington's repeated use of it on-set and in the media is simply inexcusable.
"We want to sit down with him and give him an opportunity to raise public awareness about the destructive impact of these kinds of anti-gay slurs.
"Over the past four months Washington has become part of a significant problem of anti-gay prejudice in this country. He now has an opportunity to be part of the solution."
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