Experimental Junin virus infection of a New World primate, Callithrix jacchus, was evaluated. The virus produced anorexia, loss of weight, thrombocytopenia, leukopenia, and hemorrhagic and neurological symptoms and terminated in death. Virus was recovered from urine, blood samples and all tissues taken at autopsy. These preliminary observations show that several aspects of the experimental disease in C. jacchus are quite similar to severe natural Argentine hemorrhagic fever of man.