That adhesion molecules play a major role in the regulation of normal hematopoiesis is suggested by the abundance of these molecules present on early bone marrow progenitor cells and their differential pattern of expression at discrete stages of differentiation along the various cell lineages. In particular, precursor cell matrix/endothelial interactions determine retainment or release of hematopoietic cells from the bone marrow microenvironment. Consequently, changes in the affinity or quantitative expression of adhesion molecules on either the bone marrow stroma or the blood cell precursor component-during normal development or due to activation or a malignant process-will affect cell attachment. Adhesion molecules, therefore, are modulator molecules which alter the biological behavior of normal or leukemic hematopoietic cells, primarily in terms of migration and localization properties, although they also participate in many other cell functions such as cytotoxicity, antigen presentation and binding of viruses or cancer cells. Several membrane-bound adhesion molecules and, in some instances, their soluble counterparts which may be biologically active, have been described in acute myeloid leukemia. The potential diagnostic or physiological significance of leukocyte antigens with adhesive properties will be addressed in this comment.