Herpesvirus saimiri (HVS) induced persistent, clinically inapparent infections of long-term duration in capuchin monkeys (Cebus albifrons). The infections were characterized by development of antibody to HVS-associated antigens and recovery of low levels of virus-genome-carrying lymphocytes in the peripheral blood. Peripheral lymphocyte counts remained in low-normal to normal ranges and no physical signs of lymphoma were evident. Prednisolone treatment caused immunosuppression in one monkey; this was accompanied by a progressive loss of humoral antibody to HVS-associated antigens, but neoplastic disease did not develop.