At this point Lady Macbeth enters, carrying a candle, and we soon learn why her gentlewoman is afraid to repeat what she has heard. In her sleep, Lady Macbeth relives the crimes that she has helped Macbeth to commit. First she rubs her hands as though washing them. The gentlewoman explains that she has seen the lady do this for as much as fifteen minutes at a time. Now, after rubbing her hands, Lady Macbeth looks at them and says, "Yet here's a spot" (5.1.31). What she is seeing in her trance-like state is a spot of blood that she cannot wash off her hand. We can see the irony, because just after the murder of Duncan, the lady scorned her husband for staring at his own bloody hands, and she told him that a little water would fix everything.
She continues to "wash" her hands until she is interrupted by the memory of the bell that she herself rang to summon her husband to the murder of King Duncan:
Out, damned spot! out, I say!--One: two: why,
then, 'tis time to do't.--Hell is murky!--Fie, my
lord, fie! a soldier, and afeard? What need we
fear who knows it, when none can call our power to
account?--Yet who would have thought the old man
to have had so much blood in him? (5.1.35-40)
Lady Macbeth had thought that once her husband was king, it wouldn't matter who knew that they murdered King Duncan, because no one would be able to challenge Macbeth's power as king, to "call our power to account." Yet the old man had a lot of blood, and she can still see it on her hands, reminding her of her guilt. His blood is pursuing her in another way, too, although she may not know it. A man's "blood" is his family, and Malcolm, who is of King Duncan's blood, is now marching with ten thousand English soldiers to call Macbeth to account.
Lady Macbeth's mind wanders to other horrors, and back to the blood on her hands. She asks, "The thane of Fife [Macduff] had a wife: where is she now?" (5.1.42-43), and then she wonders if her hands will ever be clean. She tells her husband to be calm, and then she smells blood on her hands and says, "Here's the smell of the blood still: all the perfumes of Arabia will not sweeten this little hand. O, O, O! (5.1.50-52).