Intramuscular or aerogenic inoculation of baboons with Lassa virus reproduced a severe form of the disease which clinically, pathoanatomically and virologically resembles the sever form of human Lassa fever reported in the Iiturature. The severity of monkey infection is demonstrated by rapid development of symptoms of general toxicity in the presence of fever, manifestations of hemorrhagic diathesis, high level of viremia and isolation of the causative agent from nasopharyngea washings.