Korean haemorrhagic fever (KHF) (Hantaan virus), a rodent-borne viral illness, is an important cause of human disease throughout much of Asia and Eastern Europe. The agent responsible for KHF has not yet been conclusively identified. Plaque-purified KHF virus was concentrated and then banded in a potassium tartrate gradient. Material from the 1.17-1.19 g/ml band was examined by electron microscopy and particles with a morphology identical to that of the family Bunyaviridae were found. The particles were aggregated by KHF serum but not by saline solution or non-immune serum. Identification of KHF virus as a member of the family Bunyaviridae implies a potential for spread by arthropod vectors.