The response of Erythrocebus patas monkeys experimentally inoculated by the intravenous and subcutaneous routes with Orungo virus (family Reoviridae, genus Orbivirus) was studied with reference to development of clinical signs, circulation of virus and antibody response. None of the animals showed clinical disease nor did they circulate virus. However, all the animals developed complement fixing (CF), neutralizing (N) and agar gel (AG) precipitating antibodies between day seven and day 14 post infection (p.i.). The CF antibodies appeared earlier and lasted for a longer period than did the N antibodies. The presence of transient 2-mercaptoethanol (2-ME) sensitive CF antibodies was demonstrated in sera collected between day seven and day 14 p.i. The significance of these findings in the interpretation of serological surveys in man for Orungo virus antibody is discussed.