Reactivation of Toxoplasma gondii infection often occurs concurrently with active cytomegalovirus (CMV) disease in immunocompromised patients, and CMV disease results in immunosuppression. To determine if murine CMV (MCMV) infection decreases resistance to T. gondii, mice with latent T. gondii infection were infected with MCMV. T. gondii infection reactivated, manifested primarily as pneumonia. Lung macrophages supported the growth of T. gondii before, during, and after T. gondii reactivation. Peritoneal macrophages inhibited the growth of T. gondii as pneumonia developed and became permissive as pneumonia resolved. Mice with latent T. gondii infection could survive larger doses ofMCMV than could controls. Thus, MCMV infection led to reactivation of latent T. gondii infection in mice. Activation of lung macrophages, assessed by their ability to inhibit replication of T.gondii in vitro, wasnot associated with control of T.gondii infection.