Hepatitis B core antigen (HBcAg) and hepatitis B surface antigen (HBsAg) were localized in human liver tissues by the peroxidase-labeled antibody method at the light and electron microscopic levels. Several methods of fixation, staining, and inhibition of endogenous peroxidase activity were studied. The periodate-lysine-paraformaldehyde fixative effectively preserved the tissue structure and the antigenicity of both antigens, and the peroxidase-labeled Fab' fraction of IgG penetrated well into hepatocytes. HBcAg was present in nuclei, or cytoplasm of hepatic cells, or both. In nuclei, the antigen was found both in virus-like particles of approximately 20 nm. diameter and in nuclear ground substance. In the cytoplasm, the antigen was found on membrane-bound ribosomes and free polysomes, and also in the ground substance of the cytosol near ribosomes and around nuclear membranes, especially near nuclear pores. HBcAg-positive virus-like particles were also demonstrated sparsely or in clusters in the cytoplasm. HBsAg was not present in nuclei but was found in the perinuclear space and in cisternae of endoplasmic reticulum, and on nuclear, endoplasmic reticulum, and cell membranes of hepatic cells. HBsAg-positive 25- to 30-nm. wide tubular forms, round particles (probably cross-sections of tubular forms), and a few large particles of 40 to 50 nm. diameter were seen in cisternae. Such HBsAg-positive particles were also present in the intercellular space and in Disse's space. These findings suggest that HBcAg produced on the cytoplasmic ribosomes migrates through nuclear pores to the nucleus and is assembled into core particles there. These particles may then move through nuclear pores to the cytoplasm where they are invested with HBsAg-positive membrane in cisternae of endoplasmic reticulum or as they enter the endoplasmic reticulum. These virus particles are then released together with other HBsAg-positive forms into the intercellular space by reversed phagocytosis.