Advances in fluorescence-activated cell sorter technology have brought about multicolor analysis of cell phenotypes. To clarify the phenotypes of human hematopoietic stem cells (HSCs), we initially prepared novel antibodies against CD34 and labeled one of them (4A1) with allophycocyanin (APC). With this, we analyzed the phenotypes of CD34+ HSCs and showed that primitive HSCs or CD34+CD33- cells expressed adhesion molecules such as CD43, CD44, CD11a, CD11c, CD18, and leukocyte adhesion molecule (LAM-1). The more primitive hematopoietic cells or CD34+CD38- cells also expressed CD11a and CD18 with an incidence of 20% to 30%. To clarify the role of adhesion molecules in HSCs, we examined the colony forming capacity after long-term culture with allogeneic irradiated stromal layers. Among CD34+CD33- cells, CD18+ cells gave rise to colony-forming cells (CFCs) on stromal layers, but reached a maximum at week 2, after which the number of generated CFCs decreased. On the other hand, CD18- cells generated less CFCs than CD18+ cells at 2 to 3 weeks, but increased after 4 weeks of culture. When CD18 or CD11a antibody was added to a coculture system of CD34+CD33- cells with stromal layers, the number of generated CFCs decreased significantly compared with the no antibody control. Leukocyte function-associated antigen-1 (LFA-1) (CD11a/CD18) was expressed on some populations of hematopoietic cells and contributed to the proliferation by interacting with stromal cells. However, more primitive cells capable of reconstituting hematopoiesis did not express LFA-1. These data provide a rationale for the administration of anti-LFA-1 antibody after bone marrow transplantation for reducing the graft failure.