An avidin-biotin immunoperoxidase procedure was optimized for detection of canine adenoviral antigens in formalin-fixed, paraffin-embedded liver. Long-term stability of viral antigen was shown by successful demonstration of virus in liver tissue preserved up to six years from dogs with infectious canine hepatitis. This immunohistochemical stain was applied to sections from livers with a wide range of inflammatory lesions. Examination of sections from 53 dogs yielded five livers with small amounts of adenovirus. An additional virus-positive liver was identified from a dog with no hepatic inflammation. Although a cause and effect relationship remains to be determined, these findings suggest a possible connection between canine adenovirus and spontaneous chronic hepatitis.