After intracerebral inoculation of Theiler's murine encephalomyelitis virus (TMEV), certain mouse strains develop a persistent central nervous system (CNS) infection and inflammatory demyelinating lesions containing infiltrates of mononuclear cells and macrophages. Previous findings demonstrating a strong correlation between disease incidence, the presence of particular H-2 region genotypes, and development of high levels of TMEV-specific delayed-type hypersensitivity (DTH) supported an immune-mediated basis for myelin breakdown. These findings led us to examine whether a possible causal relationship would be supported by a temporal analysis comparing the onset of clinical disease and the development of TMEV-specific cellular or humoral immune responses in susceptible and resistant strains. In susceptible SJL/J mice, TMEV-specific DTH and T cell proliferative (Tprlf) responses developed within 10 to 14 days postinfection, preceded the onset of clinical signs, and remained elevated for 6 mo. In contrast, resistant BALB/c mice developed low levels of TMEV-specific Tprlf and no measurable DTH. However, both strains attained comparable levels of TMEV-specific serum antibody responses with parallel kinetics. Both DTH and Tprlf responses in susceptible SJL/J mice were shown to be specific for TMEV and mediated by L3T4+, Lyt-1+2-, class II-restricted T cells. A model is proposed implicating an effector role for TMEV-specific DTH, wherein lymphokine release by virus-specific DTH T cells leads to the recruitment, accumulation, and activation of macrophages in CNS tissue, which cause bystander myelin destruction and provide a permissive population of host cells for TMEV persistence.