Balb/c mice were infected with murine herpesvirus (MHV-72) and subjected to immunosuppression (IS) with the antibiotic FK506 either during the acute or chronic phase of infection. Attempts to detect virus in various organs of immunosuppressed and non-immunosuppressed mice at different time interval were made. In the mice immunosuppressed on days 3-23 p.i. of the acute phase of infection virus was detected in various tested organs and tissues at 2.0 times higher rate than those of control mice. At later intervals of the acute phase of infection (56-84 days p.i.) virus was still recovered in bone marrow and lymph nodes peritoneal cells of immunosuppressed mice, but not in those of control animals. In the chronically infected mice immunosuppressed on days 290-320 p.i., the virus was detected in lungs, thymus, bone marrow, spleen and peritoneal cells at 3.5 times higher rate than in those of control mice.