Antiviral mechanisms by which natural killer (NK) cells control murine cytomegalovirus (MCMV) infection in the spleens and livers of C57BL/6 mice were measured, revealing different mechanisms of control in different organs. Three days postinfection, MCMV titers in the spleens of perforin 0/0 mice were higher than in those of perforin +/+ mice, but no elevation of liver titers was found in perforin 0/0 mice. NK cell depletion in MCMV-infected perforin 0/0 mice resulted only in an increase in liver viral titers and not in spleen titers. Depletion of gamma interferon (IFN-gamma) in C57BL/6 mice by injections with monoclonal antibodies to IFN-gamma resulted in an increase of viral titers in the liver but not in the spleen. Analyses using IFN-gamma-receptor-deficient mice, rendered chimeric with C57BL/6 bone marrow cells, indicated that in a recipient environment where IFN-gamma cannot exert its effects, the depletion of NK cells caused an increase in MCMV titers in the spleens but had little effect in the liver. IFN-gamma has the ability to induce a variety of cells to produce nitric oxide, and administrating the nitric oxide synthase inhibitor N(omega)-monomethyl-L-arginine into MCMV-infected C57BL/6 mice resulted in MCMV titer increases in the liver but not in the spleen. Taken together, these data suggest that in C57BL/6 mice, there is a dichotomy in the mechanisms utilized by NK cells in the regulation of MCMV in different organs. In the spleen NK cells exert their effects in a perforin-dependent manner, suggesting a cytotoxic mechanism, while in the liver the production of IFN-gamma by NK cells may be a predominant mechanism in the regulation of MCMV synthesis. These results may explain why the Cmv-lr locus, which maps closely to genes regulating NK cell cytotoxic function, confers an NK cell-dependent resistance to MCMV infection in the spleen but not in the liver.