An ultrastructural examination of livers from newborn mice, injected with potent partially purified or highly purified mouse interferon or with lymphocytic choriomeningitis (LCM) virus, has revealed the presence of tubular aggregates associated with the granular endoplasmic reticulum in the cytoplasm of hepatocytes after either treatment. Thus the lesion was observed in A2G and Swiss mice after interferon injections. It was also seen in C3H mice after LCM infection, the liver being examined at a time when the interferonaemia in the injected mice was known to be at its peak. The aggregate resembles the tubular systems associated with the endoplasmic reticulum described in various tissues in both human and animal diseases. These observations raise the possibility that in some of the cases previously described the lesion has been interferon induced.