Thymocytes of mice deficient in the recombinase-activating gene (RAG)-1 or RAG-2 cannot express and receive signals through the pre-TCR. As a result, thymocyte development in these mice terminates at the CD4/8 double negative (DN), IL-2R-alpha-positive stage. Nevertheless, RAG-deficient DN thymocytes express functional CD3 complexes and can therefore be induced by anti-CD3 epsilon mAb to mature to the CD4+8+ double positive stage. In the present paper we demonstrate that the peripheral lymphoid organs (lymph nodes, spleen) and peripheral blood of RAG-deficient mice harbor an immature T cell population which, similar to RAG-deficient DN thymocytes, contains high levels of cytoplasmic CD3 epsilon and responds to anti-CD3 epsilon mAb in vivo. With respect to surface phenotype (Thy1.2+, PgP-1+, HSA+, Fc gamma RII/III-, IL-2R-alpha-, c-kit-), these cells are similar to intermediate stage RAG-deficient DN thymocytes. Moreover, they express mRNA for pre-TCR-alpha and for the nondeleted RAG. Following injection of anti-CD3 epsilon mAb, these cells proliferate, down-regulate heat stable Ag and PgP-1, and partially differentiate to CD4+ and CD8+ double positive and single positive cells. The induced population displays a mixed phenotype, between that of immature thymocytes and lymph node T cells in normal mice. Induction is successful in thymectomized RAG-deficient mice, suggesting that it occurs in the periphery. However, after thymectomy, inducible cells disappear with an approximate half-life of 10 to 14 days. We suggest that DN thymocytes can emigrate and repopulate peripheral lymphoid organs of RAG-deficient mice. These cells respond to CD3 signaling by aberrant maturation, possibly due to the inappropriate microenvironment of peripheral lymphoid organs.