Inoculation of human serums or plasmas obtained during the early acute phase of viral hepatitis induced chemical and morphological hepatic disease in marmosets in two out of five experimental series. The disease was transmissible in series from marmoset to marmoset with an apparent increased virulence of the causative agent in later marmoset passages. The chemical evidence for the disease was elevation of the activity of SGOT and SICD and of serum bilirubin. In serial liver biopsy specimens interpreted under code, a hepatitis, exhibiting some of the characteristics of human viral hepatitis, was readily distinguishable from nonspecific changes. The morphological changes preceded the biochemical alterations and persisted after them. The data reported in these studies indicate that marmosets may be susceptible to human hepatitis. If these observations are confirmed, these animals may provide good experimental models for this disease. Final proof that the hepatitis observed in marmosets is caused by agents of human viral hepatitis is still lacking.