Hantaan virus, strain 76-118, was propagated to high titer in a clone of Vero cells, and infectious virions were successfully concentrated and purified. Infectivity and virus antigenic activity were closely associated with a virus particle that exhibited a sedimentation rate indistinguishable from a representative member of the Bunyaviridae. Purified virions sedimented to a density of 1.16-1.17 in sucrose and 1.20-1.21 in cesium chloride. Detergent disruption of virions resulted in a nucleocapsid structure (density, 1.18 in sucrose and 1.25 in cesium chloride) and soluble protein antigens. Three separate nucleocapsids were resolved by rate-zonal centrifugation and contained a single but common polypeptide of 50,000 daltons. Electrophoresis of radiolabeled RNA extracted from purified virions yielded a profile of three RNA species with apparent molecular weights of 2.7, 1.2, and 0.6 X 10(6). These data support earlier electron microscopy reports which suggested that Hantaan virus has characteristics similar to some members of the virus family Bunyaviridae.