Infection of mice with mouse hepatitis virus (MHV) strain A59 results in acute encephalitis, hepatitis, and chronic demyelinating disease. T lymphocytes play an important role in MHV infection, and costimulatory signals are an important component of T cell function. To elucidate the role of the main costimulatory molecule, CD28, in MHV pathogenesis and demyelination, we examined the kinetics of MHV-A59 infection in CD28 knockout mice. MHV-A59-infected CD28 knockout mice developed acute encephalitis and hepatitis, and the same degree of chronic demyelination as normal C57Bl/6 (B6) mice. Thus, CD28, the costimulatory T cell molecule, is not required for MHV infection and MHV-induced demyelination.