The isolation and purification of the naked intranuclear particles ofa human liver infected with hepatitis B virus is described. The particles had a diameter of 27-28 nm, were hexagonal, and showed the presence of capsomeres and spikes with lengths of 7-10 nm projecting from the surfaces. The particles formed cubic arrays with center-to-center distances of40-42 nm. The overall particle symmetry was icosahedral. The ultraviolet absorption spectrum of the particles showed a peak at 264 nm with a shoulder at 280 nm. Endogenous DNA polymerase activity was not detected in fractions containing particles. DNA polymerase activity was present when activated DNA or d(A-T). was used as exogenous template.The first specific biochemical trace of hepatitis B virus (HBV) infection was the discovery of hepatitis B antigen (HBAg) in the serum of infected patients (1). HBAg was observed to be particulate, consisting of two main morphologic forms (2): small round forms 18-20 nm in diameter and tube forms with a diameter of 20 nm, lengths of 50-230 nm, and cross striations with a periodicity of 3 nm (3). Dane et al. (4) described larger particles, 40-42 nm in diameter, with a double shell and an inner core resembling a viral nucleoid. These particles also had hepatitis B antigenic determinants on their surfaces. Dane et al. (4) suggested that these larger particles may represent the complete virion and that the other morphologic forms of particulate HBAg represented excess coat protein. A new antigen-antibody system in the sera of patients with HBV infection was found by treating pellets prepared from sera particularly rich in Dane particles with 0.5% Tween 80 (polysorbate) in phosphate buffered saline (PBS) to release the internal component of the Dane particle (5). The core of the Dane particle had a diameter of 27 nm and was morphologically similar to rhinoviruses. This particle seemed to be identical to particles observed in hepatocytic nuclei (3, 6, 7) and liver homogenates (8) of patients with HBV infection. Antibody directed against the internal component of the Dane particle, different from antibody to HBAg, was found during acute attacks of hepatitis B and persisted for as long as 3 years (9). However, the postulated identity of the Dane core and hepatocyte intranuclear naked particle has not yet been proved.Isolation of cores of Dane particles from human serum recently has been described (10). However, these cores were obtained in small quantities by detergent treatment of Dane post-fixed and Epon-embedded liver were prepared and examined by electron microscopy using a uranyl acetate-lead citrate stain according to standard techniques (12). Frozen sections of liver were examined by the direct fluorescent antibody technique with fluorescein-conjugated antisera to HBAg, IgG, IgM, IgA and beta-1-C, beta-1-A globulin (13,14). Human antiserum to HBAg was derived from a patient with hemophilia; the fluorescein-conjugated antibody was shown previously to give both cytoplasmic and nuclear fluorescence with HBV-infected...