Approximately 80% of Calomys musculinus inoculated with an attenuated strain of Junin virus (JV) developed a lethal encephalitis. Antithymocyte serum, a potent suppressor of T-cell-mediated immunity, was studied for its effect on JVpathogenicity. Early administration of an anti-C.musculinus thymocyte serum (ACTS) to neonatal animals significantly diminished clinical disease and death and abrogated brain damage, which is usually associated with viral presence in the brain. Late ACTS administration did not modify the pattern of JV infection. These results suggest that immune mechanisms participate in the pathogenesis of JV infection for its main natural host.