Expansion of stem cells from cord blood has been demonstrated to increase the numbers of CD34+ cells, CD34+ subsets, long-term culture-initiating cells, and severe combined immunodeficient mouse, repopulating cells. However, reports suggest that the ex vivo expanded population behaves differently than freshly isolated cells and shows a delayed or diminished engraftment. In this study, we investigated the effects of the cytokines flt3 ligand, stem cell factor, and thrombopoietin on expansion of CD34+ and CD34+/CD38- cells. In addition, we studied the expression of adhesion molecules, very late activation antigen-4 (VLA-4) and leukocyte function antigen-1 (LFA-1), on CD34+ cells from cord blood by flow cytometry. We also looked at the expression of an adhesion receptor, namely, vascular cell adhesion molecule-1 (VCAM-1) on bone marrow stromal cells by Western blot analysis after exposure to low dose gamma irradiation. After culturing for 7 days, increases in the absolute numbers of CD34+, CD34+/CD38-, CD34+/VLA-4+, and CD34+/LFA-1+ cells were 5.67 +/- 2.91 (mean +/- standard deviation) fold, 7.21 +/- 4.38 fold, 99.56 +/- 101.5 fold, and 101.39 +/- 83.25 fold, respectively. There was a transient upregulation in the expression levels of VCAM-1 on stromal cells, which peaked at 4 hours. Though there was an increase in the absolute numbers of CD34+ cells expressing the adhesion molecules, the expression levels (antigen density) of the adhesion molecules on the CD34+ cells remained unaffected.