Three chimpanzees were inoculated with an infectious factor VIII preparation. Two of the chimpanzees received in addition a human immunoglobulin preparation as used for prophylaxis in humans. All three chimpanzees developed an acute limited non-A, non-B hepatitis as judged from light and electron microscopic markers after an incubation period of two weeks. The use of immunoglobulin did not prevent the infection. A prolonged incubation of 15 weeks, however, was observed in one animal when alanine aminotransferase (ALT) elevation was used as criterion of infection. In the electron microscope, non-A, non-B hepatitis was characterized by tubular structures, spongelike inclusions and attaching curved membranes, in the absence of nuclear viruslike particles. An additional finding were viruslike particles in crystalline arrays which were found in the cytoplasm of sinusoidal-lining endothelial cells and tubuloreticular complexes.