The roles of the inflammatory cytokines tumour necrosis factor-alpha (TNF-alpha), interleukin-l (IL-I) and IL-12, in murine cytomegalovirus (MCMV) disease were investigated in susceptible BALB/c and resistant C57BL/6 mice. MCMV infection induced IL-1 and TNF-alpha production by peritoneal cells from BALB/c mice, as demonstrated previously in C57BL/6 mice. Overt ill-health and viral replication in the spleens of BALB/c mice were increased by in vivo treatment with soluble TNF-alpha receptors to inhibit the activity of this cytokine, whilst antibodies to IL-12 had a similar but more restricted effect. C57BL/6 mice were not affected by either treatment, suggesting TNF-alpha and IL-12 are not critical for natural killer cell-mediated restriction of viral replication in the spleen. Soluble TNF-alpha receptors and antibodies to IL-12 also enhanced MCMV titres and numbers of viral antigen-positive cells in the livers of BALB/c mice and TNF-alpha receptors have similar effects in C57BL/6 livers. In contrast, IL-l receptors improved the health of MCMV-infected BALB/c mice and reduced viral replication and hepatitis at some time-points. Mechanisms which may underlie these changes are discussed.