The vast majority of CD4+ T cells infiltrating into gastric mucosa (GM) and in the draining (gastric) lymph node (GLN) shows an activated/memory phenotype, CD45RB(low) L-selectin(low) CD44(high), in neonataly thymectomized BALB/c mice bearing autoimmune gastritis (AIG), indicating that these cells are actively involved in this disease. CD4+ T cells sort-purified from GLN expressed mRNAs encoding for both IFN-gamma and IL-4. However, those infiltrating into GM expressed very low levels of IL-4 mRNA, even though they strongly expressed IFN-gamma mRNA. Among CD4+ T cells separated from AIG mice expressing detectable levels of either IFN-gamma or IL-4 by intracellular staining, less than one-seventh expressed IL-4 and thus most of them expressed IFN-gamma in GM, whereas roughly half and one-third expressed IL-4 in GLN and spleen respectively. These findings indicate that the Th1 cells predominantly infiltrate into autoimmune lesions and Th2 cells are mainly resident in the regional LN. We further set up an in vitro model system of transendothelial migration using a murine endothelial cell line, F-2, and found that Th1 cells in CD4+ T cells separated from lymphoid tissues of AIG mice preferentially passed through the monolayer of endothelial cells while only a small portion of Th2 cells did so. This differing ability of transendothelial migration and localization might explain the dominance of Th1 cells destroying the tissue in focal lesions without inhibition by the Th2 cells, in spite of both subsets being simultaneously activated in AIG mice, and the functions of each T cell subset seems to be mutually exclusive.