When Deckard pursues the replicant Zhora, a snake scale he finds in a drain eventually leads him to a cabaret where he finds her performing a seductive dance with a snake. (It's not a "real" snake: since so many animals have died out, it's a replicant serpent itself.) Zhora herself has a snake tattoo on her neck.
So what? Is that it? Maybe… But you could argue that the snake fits in with the mythological and biblical echoes in Blade Runner, along with the pyramid or ziggurat where Tyrell lives and the "Methuselah syndrome" from which Sebastian suffers. When an unseen emcee introduces Zhora at the club, he makes reference to "the serpent that once corrupted man."
Everyone in this movie is living in a corrupt society, polluted and filthy and dominated by greedy and unscrupulous people, and the replicants get the worst of it: they're the true victims of the movie, used as slave labor and then murdered.
The Christ-imagery at the end of the movie plays into this concept, too—for example when Roy drives a nail through one of his hands and then releases a dove as he dies. He's making up for the sins of his world, while he himself is guiltless. So, you could interpret the serpent imagery as something that shows that Zhora—and the other replicants—are bearing a burden unjustly forced on them by the world's corruption.