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To examine whether Indian monkeys are infected with hepatitis E virus (HEV) in nature, serum samples from wild rhesus (Macaca mullata), bonnet (M. radiata) and langur (Presbytes entellus) monkeys were screened for anti-HEV IgG antibodies in recombinant antigen-based ELISA assays. The positivity rates were 36.7%, 19.1% and 2% respectively. The protection of such antibodies against human HEV was studied in four rhesus monkeys. Of the two rhesus monkeys with anti-HEV titres of 100 and 1000 respectively which were inoculated with the KOL-91 strain of HEV, the former demonstrated a 10-fold rise in anti-HEV titres. Anti-HEV titre in the second rhesus monkey remained unchanged. Neither of the monkeys showed any rise in serum alanine transaminase (ALT) or presence of virus in the faeces, as tested by polymerase chain reaction (PCR). Two other rhesus monkeys with anti-HEV titres of 10,000 and 100 respectively were inoculated with the AKL-90 strain of HEV. Serum ALT levels and anti-HEV titres remained unchanged in the first monkey. Excretion of virus in faeces was not noted (PCR). The second monkey developed a typical HEV infection. HEV infection could be produced in anti-HEV negative control monkeys inoculated with both strains of HEV. These results show that either human or simian HEV, or a closely related agent, is circulating among Indian macaques. Titre-dependent protection of naturally occurring anti-HEV antibodies supports this view.