The effect of murine rIL-4 on the development of herpesvirus encephalitis (HSE) in mice infected intranasally with herpes simplex virus type 1 (HSV-1) was investigated. The mortality rates of mice infected with a 0.5 LD50 dose of HSV-1 were greatly increased after the administration of rIL-4 at doses ranging from 0.01 to 1.0 U/mouse 2 h before and 2, 4 and 6 days after the infection. In contrast, survival rates of mice exposed to a 5 LD50 dose of HSV-1 were clearly increased when these mice were treated with anti-IL-4 mAb. Cervical lymph node (CLN) cells and cerebrospinal fluid (CSF) cells from mice with HSE (HSE mice) produced IL-4 in their culture fluids when they were stimulated in vitro with HSV-1 Ag. Increased amounts of HSV-1 infection in mice resulted in the increased production of IL-4 in the culture fluids of local lymphocytes. However, significant amounts of IL-4 were not produced in serum specimens or in culture fluids of spleen cells from HSE mice. IL-4 production in culture fluids of CLN and CSF cells from HSE mice was clearly reduced after treatment of HSE mice with anti-IL-4 mAb. Furthermore, IL-4 production by CLN and CSF cells was greatly enhanced when the cells were prepared from HSE mice previously treated with rIL-4. The IL-4 was mainly produced from CD4+ T cells. These results demonstrate that small amounts of exogenous IL-4 increase the severity of HSE in HSV-1-infected mice through the increased production of IL-4 from local CD4+ T cells.