Ultrastructure of liver biopsy specimens obtained from normal and hepatitis B (HBV) and hepatitis C virus (HCV) infected livers of patients and chimpanzees were compared. Nuclear alterations (glycogen particles, nuclear bodies, "vermicellar bodies", etc.), intracytoplasmic crystalloid inclusions were observed before and after the HBV and HCV infections both in human and chimpanzee hepatocytes, however, some of them were more common during the viral infection. Extreme endoplasmic reticulum dilatation characterized the human, while the presence of membranous cytoplasmic inclusions the hepatocytes of chimpanzees during HCV infection. Interferon-associated membrane alterations were noted during acute or chronic hepatitis, however, in slightly different forms in humans and chimpanzees. Data suggest to be precautions in the interpretation of the ultrastructural alteration observed in different species even to be so closely related as humans and chimpanzees especially during infection with hepatotropic viruses.