Immunostaining of the cytomegalovirus (CMV) pp65 antigen in the nucleus of peripheral blood leukocytes is a highly specific tool for diagnosis of active CMV infection. In the present study, the significance of cytoplasmic staining of leukocytes, observed in some patients with active CMV disease, was investigated. The ring-like appearance of cells with stained cytoplasm was shown to be nonspecific and inducible by incubation of blood leukocytes with interferon-gamma in vitro. Thus, only nuclear staining should be considered diagnostic in the CMV pp65 antigen test.