The production of tumour necrosis factor (TNF)-α, interleukin (IL)-1 and IL-6, all proinflammatory cytokines, was investigated in radiation-immunosuppressed rats infected with rat cytomegalovirus (RCMV). At day 7 post-infection, when the animals showed disease signs, high TNF-α levels were detected in the serum and in homogenates of various organ tissues. In contrast, IL-1 and IL-6 levels were not significantly elevated. Moreover, replication of RCMV induced TNF-α expression in different types of cells grown in vitro. When frozen tissue sections were examined by immunohistology, TNF-α-producing cells were found in areas with extensive pathology in the lungs, spleen and liver. Both lymphocytes and RCMV-infected cells were identified as the sources of TNF-α. Its abundance in RCMV-infected rats suggests an important role for TNF-α in CMV pathogenesis.