Cytomegalovirus (CMV) causes severe opportunistic infection in immunocompromised hosts. The importance of conventional alphabeta T cells in protection against CMV infection has been well documented. However, the role of the second T-cell population (which express the gammadelta T-cell receptor) in CMV infection is not known. In the present study, we analysed the function and protective role of gammadelta T cells in a murine cytomegalovirus (MCMV) infection model. After intraperitoneal infection with MCMV, the number of gammadelta T cells increased in the liver and peritoneal cavity from day 3, and reached a peak on day 5. The gammadelta T cells showed an activated T-cell phenotype and predominantly expressed Vgamma1, which is known to be expressed by heat-shock protein 65 (hsp 65)-specific gammadelta T cells. Analysis of cytokine expression demonstrated that the MCMV-induced gammadelta T cells expressed interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) but not interleukin-4 (IL-4), implying their participation in the cell-mediated immune response against MCMV. Depletion of gammadelta T cells by anti-T-cell receptor (TCR) gammadelta monoclonal antibody (mAb) treatment resulted in significant increase of virus titre and decrease of IFN-gamma in the liver on day 3 after MCMV infection, which further supports the importance of gammadelta T cells in early protection against infection. Finally, the MCMV-induced gammadelta T cells produced IFN-gamma in vitro in response to hsp 65. Our results suggest that gammadelta T cells participate in early protection against MCMV infection through recognition of hsp 65 and production of IFN-gamma.