Tissue sections from 42 specimens of liver were examined by indirect immunofluorescence microscopy for the presence of hepatitis B antigen (HB Ag). In all cases the serologic status of HB Ag was known. Fourteen of the specimens were also examined by electron microscopy. In four biopsies from three patients positive cytoplasmic fluorescence was detected using antisera prepared in animals and 20-nm. nuclear particles were found by electron microscopy. These patients were all seropositive for HB Ag, all had chronic aggressive hepatitis or active cirrhosis, and all were receiving immunosuppressive therapy at the time of examination. Nuclear fluorescent staining was demonstrated when one of these biopsies was re-examined using a human antiserum.