African green monkeys (Cercopithecus aethiops) are highly susceptible to Bolivian hemorrhagic fever (BHF). Six monkeys were inoculated with 1,000 plaque-forming units of Machupo virus, the etiologic agent of BHF. They were observed and monitored for clinical signs, body temperature, viremia, hematologic changes, and virus neutralizing antibody. Onset of fever, anorexia, and depression was noted on days 3 to 6 postinoculation. These and other signs increased in severity and all monkeys died: 5 of 6 died by day 13 and one survived until day 24. The median time to death for the group was 12.5 days. The mean value for hematocrit determinations gradually decreased to 30 on day 10 but subsequently increased. Mean neutrophil and lymphocyte values increased slightly until day 3, and then decreased to minimal values of 3,000 and 2,000, respectively, on day 10. Four monkeys were viremic by day 7 and all were viremic on day 10. The monkey that survived until day 24 had a neutralizing antibody titer of 1:32 on day 14 and appeared to recover from the initial acute illness by day 16. It died following onset of severe neurologic signs on day 23. BHF in the African green monkey is similar to the disease described in two species of macaques.