Characteristic pathological alterations of the liver in chimpanzees inoculated with non-A, non-B hepatitis sera have been described, but no corresponding findings have been reported in humans. Electron microscopic studies of the liver biopsy specimens of two homosexual patients with acquired immune deficiency syndrome, one without hepatitis (Patient 1) and one with chronic active hepatitis in remission (Patient 2), revealed the cytoplasmic tubular structures which are characteristic of chimpanzee non-A, non-B hepatitis. A cluster of 23 nm double-shelled particles was also seen in the cytoplasm of a hepatocyte in patient 1 who had received a blood transfusion 8 days before the biopsy. These particles were smaller than the Dane particles, Epstein-Barr virus, cytomegalovirus or herpes simplex virus, and different from hepatitis A virus particles; the antibodies to all of which are found in high concentration in acquired immune deficiency syndrome patients. These observations may reflect the morphologic findings for non-A, non-B hepatitis infection in humans.