This unit describes procedures for infecting newborn and adult mice with murine cytomegalovirus (mCMV). Methods are included for propagating mCMV in cell cultures and for preparing a more virulent form of mCMV from salivary glands of infected mice. A plaque-forming cell (PFC) assay is provided for measuring mCMV titers of infected tissues or virus stocks. In addition, a method is described for preparing the murine embryonic fibroblasts used for propagating mCMV and for the PFC assay.