Buoyant density of a recently discovered putative non-A to E hepatitis virus designated hepatitis G virus (HGV) was estimated in plasma or serum samples from three symptom-free carriers and two hepatitis patients. HGV RNA was detected by reverse-transcription polymerase chain reaction in sucrose density fractions with a low density at 1.05-1.10 g/cm3, and the density shifted to 1.23-1.26 g/cm3 after a treatment of peak fractions with Tween 80. Fractionated HGV was precipitated with antibodies to apolipoproteins, but not at all with antibodies to IgG; it was retained by affinity columns of lectins. These results indicate that the circulating HGV would be covered with lipoproteins of the host and has sugar moieties on the surface. The association of HGV with lipids would be responsible for the observed low density and prevent the binding with antibodies for viral persistence.