Knowledge of the cells in which Theiler' s murine encephalomyelitis virus (TMEV) persists is crucial to understanding the pathogenesis of TMEV-induced demyelinating disease; however, it is still uncertain whether oligodendrocytes or macrophages are the primary target for persistence. In this study, mononuclear cells (MNC) isolated directly from central nervous system (CNS) inflammatory infiltrates of TMEV-infected mice on discontinuous Percoll gradients were found to contain infectious TMEV. Macrophages appeared to be the principal MNC infected as determined by two-color immunofluorescence. Infectious center assay and double immunostaining together indicated the presence and possible synthesis of TMEV in approximately 1 in 225 to 1 in 1000 CNS macrophages, with 1 to 7 PFU produced per macrophage. On the basis of these findings, limited replication in macrophages is consistent with the total CNS virus content detected at any time during the persistent phase of the infection as well as the slow pace of the infection.