Liver tissue specimens for 43 HBAg seropositive patients and 22 HBAg seronegative individuals were investigated for the presence of HBAg with the immunofluorescence technique. Histological diagnoses were established with light microscopy on routinely stained frozen liver sections. The liver specimens of 42 out of the 43 HBAg-positive patients showed positive fluorescence. Three distinct fluorescence patterns could be observed: diffuse, focal and solitary. The diffuse pattern appeared to occur particularly in chronic aggressive or persistent hepatitis, the focal pattern was seen only in the group of patients without obvious alterations and the solitary pattern especially in acute hepatic necrosis.