Funnyman Jerry Seinfeld turned serious on US TV Monday night when he urged The Late Show's live studio audience to "stop laughing" as members chuckled at pal Michael Richards. Richards was appearing on the show, hosted by David Letterman, to address the controversy following his racist rant onstage at a Los Angeles comedy club. The comedian, who played quirky Cosmo Kramer opposite Seinfeld in cult sitcom Seinfeld, stunned his audience at the Laugh Factory on Friday after turning on two African-American hecklers. But, as he attempted to apologize to America on TV last night via satellite from Hollywood, many studio audience members thought the whole stunt was a joke. Richards said, "I lost my temper onstage... I got heckled and I took it badly and went into a rage and said some pretty nasty things to some Afro-Americans." But Seinfeld, a guest in the studio in New York, took offence when some audience members found his comments funny and began laughing. He said, "Stop laughing. It's not funny." This prompted Richards to address the chuckling: "I'm hearing your audience laugh and I'm not even sure that this is where I should be addressing the situation... I'm really busted up over this."