AbstractTwo distinct types of ultrastructural changes were found in the hepatocytes of chimpanzees infected with two forms of non‐A, non‐B hepatitis. In the type of infection that was of long incubation, there was a marked cytoplasmic derangement of the endoplasmic reticulum, with the formation of tubules, but no pathological changes in the nuclei. In the short‐incubation type of non‐A, non‐B hepatitis, induced experimentally in a chimpanzee that had recovered from the long‐incubation type of infection, nuclear alterations were found together with the presence of aggregates of particles measuring 15–27 nm in diameter. Cytoplasmic tubules were not seen in this type of non‐A,