More than 50% of acute viral hepatitis occurring in some developing countries appears to be unrelated to infection by HAV or HBV and accumulating evidence suggests that a high proportion of this non-A, non-B hepatitis (NANB) is enterically transmitted. Epidemics or outbreaks of enterically-transmitted NANB (ET-NANB) have been documented in the Soviet Union, Nepal, Burma, Pakistan, India, Borneo, Somalia, Sudan, Ivory Coast, Algeria, and Mexico. These outbreaks primarily affect young to middle-age adults and are often associated with a high mortality rate in infected pregnant women, approaching 20% in most reported epidemics. Several investigators have reported finding 27 to 34 nm virus-like particles (VLPs) in stools of acutely infected cases. Stools containing these small, non-enveloped VLPs have been shown to cause NANB in experimentally infected cynomolgus macaques, African green monkeys, chimpanzees, rhesus monkeys, and Saguinus mystax monkeys (tamarins). Infected primates have been shown to seroconvert to 27-34 nm VLPs recovered from stools of cases occurring in the Soviet Union, India, Nepal, Burma, Pakistan and/or Mexico, suggesting that ET-NANB is caused by one virus or class of serologically related viruses. The morphological features and physicochemical properties of one candidate virus are very similar to those of some human caliciviruses, a group of viruses that is normally associated with outbreaks of severe diarrhoea.