Leukocyte function associated antigen 1 (LFA-1) is an adhesion molecule indispensable in immune and inflammatory reactions, but its role in hematopoiesis remains obscure. Since LFA-1 is predominantly expressed by leukocytes, it is considered as a marker of late stage stem cell maturation when expressed on CD34+ bone marrow cells, and represents more mature hematopoietic progenitor cells. We observed that freshly isolated CD34+ bone marrow cells express LFA-1, and that the level of expression is highly variable. Interestingly, the expression of LFA-1 specific activation epitope L16 on these cells is low, even after culture. This demonstrates the LFA-1 is not activated, as was confirmed by low adhesion to ICAM-1. Culturing sorted CD34+ LFA-1+ cells in single cell per well assays in medium supplemented with SCF, Epo, IL-3, Il-6, GM-CSF, and G-CSF revealed that they gave rise to dispersed macrophage-like colonies, supporting the notion that CD34+LFA-1+ cells indeed consist of a mature committed cell population. In contrast, sorted CD34+LFA-1- cells had high proliferative potential and developed into large multilineage colonies within 14 days of culture. Unanticipated, in time course experiments we observed that these CD34+LFA-1- cells expressed LFA-1 within 24 hours upon culture. This induction was neither caused by the monoclonal antibody used to tag CD34 cells, nor dependent on growth factors present in the medium. These findings demonstrate that two populations of CD34+LFA-1+ cells can be discriminated: leukocyte lineage committed CD34+ cells in freshly isolated bone marrow cells, and multipotent CD34+ cells that acquired LFA-1 upon in vitro culture. These in vitro findings support the hypothesis that once contacts with bone marrow stroma are lost, LFA-1 is upregulated by default, due to the lack of negative regulating signals from stromal cells. This might also explain the widely variable expression of LFA-1 as a result of crowding of cells in the bone marrow with subsequent loss of contact with stroma and upregulation of LFA-1, providing those cells with adhesion receptors enabling migration in the periphery.