By using HLA markers to distinguish T cells originating in the neonatal thymus vs the CB CD34+ cells, we demonstrated that, despite exhaustive post-transplant treatment with T cell-depleting antibodies, cells carried in the neonatal thymus graft are a major source of early peripheral T cell population, and that these may destroy CB progenitor cells and may accelerate the development of GVHD/autoimmune disease. Here, CD34 is linked to graft versus host disease.