To determine whether the human thymus provides an environment for the maturation of murine T cells, human fetal thymus and liver (hu-thy/liv) were implanted into congenitally athymic NIH-beige-nude-xid (BNX) mice or C.B-17 scid/scid (SCID) mice. 3 mo after implantation, in contrast to the hu-thy/liv implant in SCID mice, which was populated only with human CD4/CD8 single- and double-positive thymocytes, the hu-thy/liv implant in BNX mice contained a chimeric population of human and mouse CD4/CD8 single- and double-positive thymocytes. Immunohistochemical staining of the hu-thy/liv implant in BNX mice indicated that the population of double-positive mouse thymocytes was localized to discrete areas of the human fetal thymus. Quantitative improvements in mouse T cell and immunoglobulin (Ig) G parameters were observed after grafting of the human fetal thymus and liver tissue into BNX mice. In addition, in contrast to the nonimplanted BNX mice, the implanted BNX mice were capable of mounting a keyhole limpet hemocyanin-specific IgG response and their peripheral T cells were responsive to stimulation with mitogens and antibodies directed to the T cell receptor. Furthermore, after in vivo priming, T cells present in lymph nodes of the implanted BNX mice were capable of mounting an antigen-induced in vitro T cell-dependent proliferative response. Thus, concurrent with the continued maturation of human T cells, murine T cells differentiated within the human fetal thymus implanted in the BNX mice and mediated the phenotypic and functional reconstitution of the murine immune system. Mice with a reconstituted immune system that contain a human thymic implant that is infectible with human immunodeficiency virus (HIV) should prove useful in the investigation of T cell maturation in the thymus and in the evaluation of potential HIV vaccines.