Viral hepatitis can occur spontaneously in chimpanzees and patas monkeys, usually producing only mild clinical symptoms which are difficult, if not impossible, to recognize without adequate laboratory studies or liver biopsy. The disease, however, can be transferred to other subhuman primates as well as to human beings by contact. The source of the infection remains obscure in most cases and the virus may be of human or animal origin. After transfer of hepatitis virus from chimpanzees to human beings, it often causes a much more severe illness than in the primates; the disease then is indistinguishable from primary viral hepatitis by clinical, laboratory and histologic means.