Recently, T cell subpopulations presumably representing memory T lymphocytes have been described in vitro. Intestinal lamina propria T cells (LP-T) have characteristics resembling those of memory cells. We therefore investigated the expression of surface Ag associated with memory phenotype in vitro on lamina propria lymphocytes (LPL) and PBL and on the T cell subpopulations defined by the bright expression of CD45R0 by flow cytometric analysis of isolated cell populations. LPL had significantly increased percentages of CD45R0 and CD58 positive cells compared with PBL. Whereas PBL showed bimodal expression profiles of CD45R0, CD58, and CD2, the vast majority of LPL was bright for these Ag. Expression of CD45RA was significantly reduced in both frequency and intensity in LPL, and LPL had significantly reduced percentages of CD11a/CD18 and CD29 positive cells compared with PBL. The CD45R0 bright T cell subpopulations of both PBL and LPL were characterized by a lack of CD45RA. CD45R0 bright T cells from the peripheral blood (PB-T) were predominantly bright for CD2, CD58, CD29, and CD11a/CD18 whereas CD45R0 dim PB-T had bimodal expression profiles and CD45R0 negative PB-T were dim or even negative for these Ag. CD45R0 bright LP-T were also bright for CD2 and CD58 but had significantly reduced surface densities of CD11a/CD18 and CD29 compared with CD45R0 bright PB-T. The surface density of CD29 on CD45R0 bright LP-T corresponded to that of CD45R0 negative PB-T, and a significant proportion of CD45R0 bright LP-T was even negative for CD11a/CD18 and CD29. Additionally, CD45R0 bright LP-T in contrast to PB-T were characterized by a lack of 1-selectin and the expression of CDw49a and the mucosa-specific T cell Ag HML-1 on high percentages of cells. Our results show that the phenotype of CD45R0 bright T cells from the lamina propria clearly deviates from that of memory T cells in vitro and of CD45R0 bright T cells in the peripheral blood. We conclude that memory T cell populations in vivo undergo specific differentiation depending on their tissue localization, leading to unique phenotypic and presumably functional features.