NFS/N mice infected with neurotropic Cas-Br-M murine leukemia virus (MuLV) at 21 days of age were resistant to neurologic disease and demonstrated MuLV-specific cytotoxic T lymphocyte (CTL) activity in spleen cells after in vitro stimulation. NFS/N mice infected with Cas-Br-M MuLV at 2 days of age failed to generate a significant MuLV-specific CTL response and developed neurologic disease 5-8 weeks later. Protection from neurologic disease transferred with fewer in vitro stimulated immune spleen cells than immune T cells from NFS/N mice infected with Cas-Br-M MuLV at 21 days of age. Cas-Br-M MuLV-specific CTL may play an important role in resistance to the paralytic effects of Cas-Br-M MuLV infection by affecting virus dissemination to the central nervous system.