MHV is probably the most widely distributed infectious disease of mice used for experimental purposes. The virus is lethal to the athymic mouse and can alter the immune system in other strains of mice. Many important gaps remain in our knowledge of the epidemiology of MHV and the biology of the virus. The objectives of our studies were to study the natural history and transmission of mouse hepatitis virus (MHV) in the animal facility setting and to gain insight into methods of control of the infection. In the past the transmission of mouse hepatitis virus (MHV) was studied in animal rooms where mice were naturally infected, using euthymic and athymic mice; and in unnatural settings where experimentally infected cage mates served as time-controlled donors. This past year we planned to repeat these studies using CD-1 and nu/nu mice. Time nor space for isolation of infected and/or exposed mice have not permitted us to repeat these transmission studies with mice experimentally infected with MHV. In addition, MHV was endemic in only one mouse room during the past year. Therefore, we had only one room for comparative transmission studies. Currently we plan to continue to collect murine viral (MHV) transmission data on a monthly schedule from sentinel CD-1 mice housed in the MHV endemic room and from other mouse rooms. These procedures are employed as a part of our sentinel animal program.