To investigate the mechanism by which viruses are cleared from neurons in the central nervous system, we have utilized a mouse model involving infection with a neurotropic variant of mouse hepatitis virus (OBLV60). After intranasal inoculation, OBLV60 grew preferentially in the olfactory bulbs of BALB/c mice. Using in situ hybridization, we found that viral RNA localized primarily in the outer layers of the olfactory bulb, including neurons of the mitral cell layer. Virus was cleared rapidly from the olfactory bulb between 5 and 11 days. Athymic nude mice failed to eliminate the virus, demonstrating a requirement for T lymphocytes. Immunosuppression of normal mice with cyclophosphamide also prevented clearance. Both CD4+ and CD8+ T-cell subsets were important, as depletion of either of these subsets delayed viral clearance. Gliosis and infiltrates of CD4+ and CD8+ cells were detected by immunohistochemical analysis at 6 days. The role of cytokines in clearance was investigated by using an RNase protection assay for interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-2, IL-3, IL-4, IL-5, IL-6, tumor necrosis factor alpha (TNF-alpha), TNF-beta, and gamma interferon (IFN-gamma). In immunocompetent mice there was upregulation of RNA for IL-1 alpha, IL-1 beta, IL-6, TNF-alpha, and IFN-gamma at the time of clearance. Nude mice had comparable increases in these cytokine messages, with the exception of IFN-gamma. Induction of major histocompatibility complex class I (MHC-I) molecules on cells in infected brains was demonstrated by immunohistochemical analyses in normal and nude mice, suggesting that IFN-gamma may not be necessary for induction of MHC-I on neural cells in vivo.