Actor Neil Patrick Harris turned the air blue while responding to a heckler during his Broadway show.

The How I Met Your Mother star appears as a transgender glam rocker in Hedwig and the Angry Inch, the cult musical written by John Cameron Mitchell.

During a recent performance one fan was unable to contain her love for the actor, interrupting the performance by shouting out, "I love you Neil!"

Unperturbed, Harris responded with a foul-mouthed retort, replying, "I'm doing something up here, ­motherf**ker!"

According to New York Post gossip column Page Six the actor's response was met with a round of applause from the audience.