A dose and time related effect on neurologic disease expression followed transfer of viral specific cytotoxic T lymphocytes (CTL) to recipient NFS/N mice previously infected at 2 days of age with Cas-Br-M murine leukemia virus. Cas-Br-M MuLV gp70 was expressed in spleen and capillary endothelial cells in the brain and spinal cord of CTL recipients, but the progression of gliosis, vacuolation, and cell death that followed endothelial cell MuLV gp70 expression in unprotected Cas-Br-M MuLV infected mice was interrupted in protected CTL recipients. A direct cytotoxic effect of CTL on infected brain capillary endothelial or neural cells could not be demonstrated. Reduced levels of infectious MuLV and MuLV gp70 expression in brain following syngeneic CTL transfer early in the course of disease suggest that CTL may function by preventing a time-limited interaction of Cas-Br-M MuLV with a susceptible target cell or receptor critical for neurologic disease induction during the perinatal period.