To develop an animal model for cytomegalovirus (CMV)-associated disease, we chemically immunosuppressed four rhesus macaques. Our immunosuppression regimen (anti-rhesus CD3 MAb, cyclosporin and cytoxan) was designed to mimic immunosuppression in human organ transplantation recipients that develop CMV-associated disease. Three of the four macaques were previously shown to be seropositive for CMV by Western blot analysis and were followed for CMV reactivation. The fourth macaque was seronegative for CMV and was experimentally infected with rhesus CMV to follow a primary CMV infection. Complete blood counts were performed on each animal prior to infection and at times thereafter to monitor hematological abnormalities. Additionally, blood samples were measured by flow cytometry to assess the effects of immunosuppression and virus infection on lymphocyte subsets (CD4/CD8/CD20). Utilizing this regimen all four macaques displayed signs of immunosuppression within the first and second weeks and sustained decrease throughout the first 14 weeks. To detect CMV viremia in a primary infection or CMV reactivation, DNA from peripheral blood leukocytes was isolated and subjected to one round of PCR amplification with oligonucleotide primers specific for the rhesus CMV immediate-early gene region. By this assay, CMV viral load was detected in the peripheral blood compartment of the macaque receiving the primary infection, while no CMV reactivation was observed within the three CMV(+) macaques. We believe there are two possibilities to potentially explain the lack of CMV reactivation in the three macaques. First, an additional stimulus may be required for CMV reactivation. In solid organ and bone marrow transplants, the organ or bone marrow is derived from an allogenic donor. Hence, a concomitant allogenic stimulus may be required for CMV reactivation. The second possibility is that the immunosuppression was not severe enough for reactivation. With acquired immunodeficiency syndrome (AIDS), CMV is known to reactivate and cause a multitude of disease complications. The acquired immunodeficiency most likely results in a more severe and sustained immunosuppression.