T cells play a critical role in hematopoietic stem cell transplantation. Many transplant protocols involving HLA matched sibling donors make use of T cell mediated graft-versus-leukemia or graft-versus-cancer effects to clear and/or prevent the recurrent growth of malignant cells. Donor T cells are also responsible for reconstituting cellular immune function following transplantation, but often cause graft-versus-host disease (GVHD). Over the past several years the major source of hematopoietic stem cells for transplantation has changed from marrow to granulocyte-colony stimulating factor (G-CSF)-mobilized peripheral blood stem cells (PBSCs). Despite the fact that patients transplanted with allogeneic G-CSF-mobilized PBSCs receive approximately 10-fold more T-cells than those transplanted with marrow, the incidence and severity of acute GVHD is similar among the two groups, although chronic GVHD is higher in PBSC recipients. Using cellular function assays some investigators have shown that G-CSF affects T cell function. When healthy subjects are given G-CSF, a decrease is seen in the concentration of circulating type 1 helper T cells (Th1) with subsequent falls in the cytokine levels produced by these cells. In contrast, an increase occurs in the concentration of type 2 helper T cells (TH2) and the level of cytokines produced. The purpose of this study is to assess G-CSF-induced changes in T cells. cDNA microarrays will be used to compare genes expressed in CD4 and CD8 cells before and after G-CSF is given. Up to fifteen healthy subjects will be given a typical PBSC mobilization regimen for allogeneic transplants: 10 micrograms/kg/day of G-CSF for 5 days. Peripheral blood mononuclear cells will be collected by apheresis before the mobilization regimen is given and the day after the mobilization is complete. CD4, CD8, and CD34+ cells will be isolated from the peripheral blood mononuclear cells and gene expression will be assessed using cDNA microarrays containing over 8,000 genes.