Investigators say the dead man, Anthony Casey, was one of five guests at the house for a late-night party early Thursday morning, The Independent reported. Another guest was detained for a few hours before being released on bail.
This is the second time Bismarck has been involved in a suspicious death. When he was a student at Oxford 20 years ago, Olivia Channon, daughter of a Conservative cabinet minister, died of a heroin overdose in his bed.
Bismarck left Oxford and was treated for alcoholism in Germany. He paid a fine for possession of amphetamine sulphate.
His ancestor, Prince Otto von Bismarck, known as "the Iron Chancellor," arranged the unification of Germany into a single state and served as chancellor from 1871 to 1890.