Fatal meningitis following intracerebral inoculation of lymphocytic choriomeningitis virus (LCMV) reflects an immunopathological lesion believed to be mediated by cytotoxic T cells. The results presented here demonstrate that pretreatment with cyclophosphamide (Cy; 150 mg/kg body weight) 2 days before intracerebral infection significantly reduced the lethality of the infection. However, this treatment did not impair the antiviral cytotoxic response as measured in the spleen. On the other hand, virus-specific delayed-type hypersensitivity (DTH) was significantly reduced. This reduction seems to be the result of a Cy-induced lack of non-committed ancillary cells since: (1) virus-primed spleen cells from Cy-pretreated donors conferred normal LCMV-specific DTH to naive recipients; (2) transfer of virus-primed spleen cells from untreated donors did not increase the suppressed DTH response of the Cy-pretreated mice; and (3) inoculation of irrelevant antigen and antigen-primed spleen cells into the footpads of Cy-pretreated, infected mice resulted in a significantly reduced footpad swelling as compared with untreated, infected controls. Taken together, these results indicate that LCMV-induced meningitis does not solely represent T-cell-mediated cytotoxicity in vivo but that a fatal outcome of the infection critically involves not only effector T cells but also ancillary cells.