T cells can be divided into two main phenotypic subpopulations-i.e., the CD45RA-positive (2H4-positive) "naive" subset and the CD45RO-positive (UCHL1-positive) "memory" subset. In light of this recent functional reinterpretation of T-lymphocyte subpopulations, we reinvestigated the composition of the inflammatory infiltrate in liver biopsy specimens from patients with acute and chronic hepatitis. In normal liver, the few scattered mononuclear cells present in portal tracts and in the intralobular parenchyma consisted of both CD45RA-positive (2H4-positive) naive and CD45RO-positive (UCHL1-positive) memory T cells. In inflammatory liver diseases, portal tract and periportal and intralobular areas of inflammation consisted virtually only of CD45RO-positive (UCHL1-positive) memory T cells, which strongly expressed the CDw29 (4B4) antigen, and the adhesion molecules LFA-1, CD2, LFA-3, CD44 and VLA-4 and the activation marker human leukocyte antigen-DR. These results indicate that activated memory T cells represent the predominant subpopulation of lymphocytes in areas of liver inflammation. Memory T cells strongly express various homing receptors and adhesion molecules, which probably allow them to accumulate at inflammatory sites and to strengthen interaction with target cells. Furthermore, the increased number of memory T cells with enhanced interferon-gamma production in areas of liver inflammation may contribute to the maintenance and up-regulation of immune responses occurring in inflammatory liver diseases.