A human agent of non-A, non-B hepatitis (Inoculum I) was transmitted to chimpanzees and alterations in liver and lymphocytes were studied by electron microscopy and by cytochemical techniques during the acute phase of the disease. Three types of cytoplasmic alterations, consisting of a membraneous and an amorphous part were observed in the hepatocytes. The density of the amorphous constituent decreased after treatment with pronase, but not after treatment with ribonuclease (RNase) or deoxyribonuclease (DNase). The wall of C-III, but not C-II had fibrils with a periodicity the contrast of which markedly increased after pronase treatment. Cytochemical data suggest that the inclusions (C-I-III) represent a cellular reaction to the infectious agent rather than the virus itself. Intranuclear vermicular inclusions (INI) were observed in hepatocytes and lymphocytes as well, mainly in degenerating cells. Tubuloreticular inclusions (TRS) did not appear in circulating lymphocytes during acute infection; however, they could be induced by human alpha interferon treatment in vitro. Increased numbers of lymphocytes with parallel tubular arrays (PTA) were noted at the peak of serum aminotransferase elevations. The latter two alterations (TRS and PTA) most likely represent immunologic reactions of the host to the infectious agent.