Extrathymic T cells in the hepatic sinusoids and intraepithelial lymphocytes (IEL) in the intestine of mice have both similar and different properties. In this study, both types of extrathymic T cells in mice were further characterized. Lymphocytes obtained from systemic immune organs, including the lamina propria and Peyer's patches, were also compared. Extrathymic T cells in both the liver and the intestine contained a large proportion of gamma delta T cells and expressed the alpha alpha homodimer of CD8. They became more prominent in athymic nude mice and in normal mice with aging, while disappearing in scid mice. Extrathymic T cells in the liver, on the other hand, had TCR of intermediate intensity (i.e., intermediate TCR cells) and IL-2 receptor beta-chains (IL-2R beta) of high intensity, similar to NK cells, whereas IEL had TCR of bright intensity and consisted of cells with both low and high levels of IL-2R beta. Thymus-derived T cells did not express IL-2R beta at all, at least at their resting conditions. Intermediate TCR cells included double-negative CD4-8- cells as well as single-positive cells. In contrast, IEL contained both double-positive (DP) CD4+8+ cells and single-positive cells. More precisely, IEL gamma delta T cells were mainly IL-2R beta + and single-positive (mainly CD8+), while IEL alpha beta T cells were mainly IL-2R beta- and contained both DP CD4+8+ cells and single-positive cells. CD4+ cells were more predominant than CD8+ cells in the liver, while CD8+ cells were overwhelmingly predominant in the intestine. These results suggest that both intermediate TCR cells and IEL are generated as primitive T cells in phylogeny, but later develop along independent pathways at their respective sites.