Intracerebral inoculation of Junin virus (JV) in all susceptible mouse models available induces a lethal meningoencephalitis compatible with a delayed-type hypersensitivity (DTH) immune response. In contrast, adult BALB/c mice prove resistant to infection and no DTH response is seen. JV inoculation in adult BALB/c mice induces DTH suppression to unrelated antigens such as sheep red blood cells. (SRBC). This suppression is mediated by JV-induced T cells (Tsv), which are operative from 1 to 24 days post-infection (p.i.), and seems to be related to adult mouse survival. The presence of JV-induced contrasuppressor cells (CS) bearing the Thy-1+, Ly 1+2- phenotype, able to abrogate Tsv cells-mediated suppression, is described here. Thus, CS cells may be involved in the mechanism by which mice avoid over-exposure to Tsv-mediated DTH suppression. Such CS cells were found in the spleen of inoculated animals and may also be induced by transferring JV-free Tsv cells to 'naive' recipients, in which JV inoculation then induces morbidity.