After administration of haploidentical stem cells to infants with severe combined immunodeficiency disease (SCID), mature T cells of donor karyotype appear later in the recipient without causing graft-vs-host disease (GVHD). To investigate the effect of the host microenvironment on these genetically donor T cells, mixed leukocyte cultures were carried out. Unfractionated mononuclear cells (MNC) from eight infants with SCID immunologically reconstituted by haploidentical bone marrow stem cells responded in the same pattern as MNC from non-chimeric individuals to autologous and allogeneic irradiated MNC, even though they contained all genetically donor T cells and all genetically patient B cells and monocytes. This included surprisingly vigorous proliferative responses of the patients' MNC to the original donors' irradiated MNC. This autoreactivity could be detected as soon as T cell function appeared post-transplantation and appeared to increase with time. It could be blocked by the addition of monoclonal antibodies to HLA Class II antigens. Responses of most patients' MNC were similar whether stimulated by irradiated MNC from the donor or non-donor parent or by those from unrelated normal controls. Purified genetically donor T cells that had matured from stem cells in the patient's microenvironment responded vigorously to purified donor B cells. These same cells responded much less to patient B cells. In six cases, such genetically donor T cells responded less to patient B cells than fresh donor T cells did to donor B cells in the autologous mixed leukocyte response. By contrast, T cells of donor karyotype from three of the patients responded more vigorously to donor B cells than fresh donor T cells did. Thus, genetically donor T lymphocytes that had matured from stem cells in the recipient microenvironment behaved differently from those that had matured in the donor. The hyporesponsiveness of genetically donor T cells from the patient to patient B cells does not appear to be due to T suppressor cells.