Poison
Poison is an important symbol in The Duchess of Malfi, as it comments on the broader political situation at the heart of the play's drama. Antonio acknowledges at the beginning of the play that a good government is like a fountain, but if that fountain is poisoned then the entire nation will descend into disease and ruin. Poison therefore becomes a symbol of corruption, which is rife among those involved in the government of Malfi.
Sickness
Sickness and disease are symbols of internal strife. When the Duchess becomes pregnant, Bosola first becomes privy to the situation when he notices her morning sickness. Then, when Ferdinand becomes mad, a doctor diagnoses him with Lycanthropia, or werewolf syndrome. In these cases, sickness becomes a physical embodiment of private guilt, shame, or secrecy.
Blood
Blood symbolizes a number of things throughout the play, but it is mostly a symbol of violence. The play is unquestionably bleak, and features a number of murders that are not only tragic but also violent. These murders are frequently described as bloody to emphasize their brutality, but also the fact that murders are being carried out by members of the same family – those of the same "blood."
Rings
Rings in The Duchess of Malfi are a symbol of fidelity, virtue, and consecrated bonds. When the Duchess gives Antonio a ring at the beginning of the play, it is a symbol of their marriage and commitment to one another. The meaning of the ring is then, for the rest of the play, perverted by the villains – the Cardinal tears the ring off the Duchess's hand in Act Three, and is later shown to the Duchess on the hand of a dead man in Act Four. The ring is in many ways a microcosm for the play as a whole, which offers a critical and despairing outlook on the nature of humanity.
The Wolf
The wolf is a symbol of savagery, violence, and bestiality. Ferdinand makes frequent references to wolves throughout the play before developing Lycanthropy, or the believe that he is frequently transformed into a wolf. Ferdinand's illness leads him to believe that he is becoming, physically, the thing he is on the inside: blood-thirsty, predatory, and violent.
