L-selectin, a surface adhesion glycoprotein expressed on leukocytes, has a well-established role in mediating inflammation and lymphocyte recirculation. Recent evidence suggests that L-selectin may also influence hematopoiesis. We observed that a greater proportion of CD34+ cells express L-selectin in cord blood compared with adult bone marrow, and we hypothesized that L-selectin expression is associated with enhanced clonogenic properties. To test this, we compared CD34+/L-selectin+ cells with CD34+/L-selectin- cells in hematopoietic clonogenic assays. From CD34+/L-selectin+ cell cultures, we observed a 3-fold increase of d 12-14 colony-forming unit-granulocyte/macrophage and multipotent progenitor cells, and a 5-fold enhancement of primitive d 21 high proliferative potential colony-forming cells compared with the progeny of CD34+/L-selectin- cells. We conclude that CD34+ cord blood cells expressing L-selectin are enriched in their clonogenic activity compared with cell fractions lacking L-selectin expression.