Mouse hepatitis virus strain JHM causes a chronic demyelinating disease in susceptible strains of rodents. Demyelination does not develop in infected RAG1-/- (recombination activation gene-deficient) mice but can be induced by several experimental interventions, including adoptive transfer of virus-specific T cells or antibodies. A common feature of demyelination in these models is extensive infiltration of macrophages/microglia into the white matter. The data obtained thus far do not indicate whether macrophage/microglia infiltration, in the absence of T cells or antibody, is sufficient to mediate demyelination. To determine whether the expression of a single macrophage chemoattractant, in the context of virus infection, could initiate the demyelinating process, we engineered a recombinant coronavirus that expressed the chemokine CCL2/monocyte chemoattractant protein-1. CCL2 has been implicated in macrophage infiltration into the central nervous system and is involved in demyelination in many experimental models of demyelination. Extensive macrophage/microglia infiltration and demyelination has developed in RAG1-/- mice infected with this recombinant virus. Thus, these results suggest that the minimal requirement for demyelination is increased expression of a single macrophage-attracting chemokine in the context of an inflammatory milieu, such as that induced by a viral infection.