Acute lymphocytic choriomeningitis virus (LCMV) infection is associated with general immunosuppression which develops during the second week of the infection and persists for several weeks. In the present study, the ability of LCMV-infected mice to mount a cytotoxic T-lymphocyte response was investigated in a transplantation assay, using LCMV-immunized mice as recipients. By this means it was possible to evaluate the T-cell responsiveness of the acutely infected mice separately. Our results revealed a marked depression of the T-cell function temporally related to immunosuppression in the intact mouse. Furthermore, this hyporesponsiveness could not be explained as an effect of suppressor cells. Occurring shortly before these changes were a drastic decrease in cortical thymocytes and a reduction in T-cell progenitors in the bone marrow and spleen. Our findings are consistent with the assumption that a numerical deficiency of immunocompetent T-cells due to viral interference with T-cell maturation plays an important role in LCMV-induced immunosuppression.