Cytomegalovirus infections are very dangerous for patients receiving an immunosuppressive therapy. Therefore, we investigated the influence of the immunsuppression, induced by cyclophosphamid onto the course of the infections with murine cytomegalovirus in mice.The disease is a suitable model for the investigation in animal experiments since many problems in human cytomegalo infections are unsolved due to the species specifity of the virus.Virus-infected NMRI mice without cyclophosphamid pretreatment showed no symptoms of the disease, cytomcgalie hepatitis was minimal and short lasting, and the mortality was 1%. All animals pretreated with cyclophosphamid and infected with the virus got sick, the mortality rate being high. After a single dose of 80 mg cyclophosphamid per kilogram body weight the mortality was 48%, after 3 doses of 80 mg/kg given prior to the infection, it was 84%. The infection of the liver lastet longer in all prctreated animals. The virus isolation was 70 times as high in animals given one dose of cyclophosphamid, and 570 times as high in those given 3 doses compared t~ animals without pretrcatment.Our experiments show clearly that even a single dose of eyclophosphamid is able to elevate the risk of infection during the immunsuppressive action, and that a relation exists between ~he effectivity of immunsuppression, mortality rate and virus multiplicatiou.