The mononuclear cells of the central nervous system (CNS) inflammatory response were characterized in cerebrospinal fluid (CSF), meningeal exudate, and brain parenchyma of mice 3-14 d after infection with Sindbis virus. The inflammatory infiltrate in CSF peaked and resolved before that of the parenchyma or meningeal exudate. Immunoperoxidase staining with monoclonal antibodies identified CSF inflammatory cells to be almost exclusively T cells, while inflammatory cells in the brain parenchymal perivascular cuffs and the meninges were a mixture of T cells, B cells, and macrophages. The percentage of B cells and macrophages increased at the later time points. Approximately 20% of CSF and 50% of the cells present early in the perivascular cuffs were not identified, suggesting that another subset of inflammatory cells may be present. We concluded that significant differences exist in the time course and cellular composition of the inflammatory responses in different compartments of the CNS during an acute viral infection.