Adoptive transfer of lymph node and spleen cells from mice infected with LCM virus to similarly infected immunocompromised recipients has been the classic way to demonstrate the lethal role of T cells in the CNS disease caused by this virus. Isolation and adoptive transfer techniques are presented here which show that Thy-1+ cells isolated from the meningeal infiltrates (MI) of LCM virus-infected mice possess this property. We compared various T cell functions of MI cells taken from mice infected with two strains of LCM virus differing markedly in their pathogenicities. One of these strains, termed aggressive, caused a typical, invariably fatal, CNS disease within 7 to 10 days after infection. The other virus, termed docile, killed few mice after the standard intracerebral inoculation, and could persist in the mice for 6 mo or more. The yields of MI leukocytes from mice infected with docile virus varied from 50 to 100% of those found in mice infected with aggressive virus (3 X 10(6) cells/brain). On a cell-to-cell basis, the CTL activity in the MI of mice infected with docile virus ranged from 50 to 100% of that found in the MI of mice infected with aggressive virus. MI cells from mice infected with aggressive virus consistently caused lethal disease by adoptive transfer into immunocompromised (irradiated) recipients infected with either strain of virus. All attempts to induce lethal disease by adoptive transfer of MI cells (or splenocytes) from mice infected with docile virus into irradiated recipients failed. The latter experiments with the docile-MI cells were performed with six times the number of aggressive-MI cells needed to kill irradiated recipients by adoptive transfer. The possible reasons for this discordance between CTL and in vivo killer function are discussed.