Lady Macbeth responds to her husband's outburst in Act III, Scene IV with a question: "Are you a man?", she asks him. Again calling into question his masculinity just as she did when he told her he did not want to kill King Duncan.
She then exclaims, sarcastically to him that he is imagining something, just like he thought he saw the dagger floating in the air before he killed Duncan. She continues to chide him about his fear and that he looks ridiculous and reminds her of a woman telling scary stories. He is spoiling the party with his nonsense.
In this scene Macbeth sees the ghost of Banquo during the banquet at his castle just after receiving confirmations that the murderer's he had hired had actually killed Banquo. First Macbeth speaks to the ghost that no one else could see. Lady Macbeth responds to this first scene of apparent madness by explaining to his guests that Macbeth has had such spells and visions since childhood and telling them to ignore him. Taking Macbeth aside, she then tries to snap him out of his spell. Macbeth makes a brief recovery until the ghost appears again. Now Macbeth appears increasingly mad and Lady Macbeth is afraid that in this state he will actually confess his crime. Thus she hurries the guests out of the hall explaining that his illness is growing worse. At this point, Lady Macbeth herself is not really sure what is going on in Macbeth's mind. She did not see the ghost herself. As the guests leave, the ghost leaves also and Macbeth seems to come back to himself somewhat worrying about Macduff's absence from the banquet. At this point Lady Macbeth urges Macbeth to get some sleep.