Very late antigen-4 (VLA-4), which binds to the extracellular matrix protein fibronectin, is an integrin molecule known to be modulated during mobilization of CD34+ cells, and to be involved in signaling the mobilization stimuli. On the hypothesis that cell cycling status might be different depending on the level of VLA-4 expression, we investigated the DNA contents of human cord blood CD34+ cells during ex vivo expansion by recombinant human thrombopoietin and flt3-ligand with simultaneous measurement of surface VLA-4 at the 1st and 4th week. During this ex vivo expansion, expression of VLA-4 increased and almost all cells became VLA-4+ until the 4th day of culture. Expression of VLA-4 was maintained in the major population of the cultured cells until the 4th week. The cells in S/G2/M phase were greater in number in VLA-4 high fraction than in VLA-4 low fraction (n=4, p<.001). Furthermore, the fraction of cells in S/G2/M phase increased as the expression of VLA-4 became higher. These results suggest that cord blood CD34+ cells expressing high levels of VLA-4 have more proliferative activities.