CD45R0 (UCHL1), a member of leucocyte common antigen family, is expressed largely on previously activated or memory T cells. We examined CD45R0 expression of T cell subpopulations in patients with Epstein-Barr virus (EBV) induced infectious mononucleosis (IMN) as a sign of in vivo activation. Consistent with the notion that activated CD8+ T cells expand in acute IMN; the majority of CD8+ T cells in patients with acute IMN expressed CD45R0 to the similar extent to HLA-DR expression. Most CD4+ T cells in these patients also demonstrated marked expression of CD45R0 as well as HLA-DR antigens, compared with age-matched controls. Expression of CD45R0 by CD4+ T cells in patients with acute IMN was more notable than their HLA-DR expression. While predominant CD8+ T cells resulted in decreased percentages of CD4+ T cells, CD4+ T cells expressing CD45R0 were shown to be significantly elevated in absolute number. The results suggest that both CD4+ and CD8+ T cells may be activated by stimulation with EBV infection. The appearance of two T cell subpopulations expressing CD45R0 in acute IMN implies their immunoregulatory roles in the control of EBV-infected cells.