W e studied the adhesion of primitive and committed progenitors from chronic myelogenous leukemia (CML) and normal bone marrow to stroma and to several extracellular matrix components. In contrast to benign primitive progenitors from CML or normal bone marrow, Ph1-positive primitive progenitors from CML bone marrow fail to adhere to normal stromal layers and to fibronectin and its proteolytic fragments, but do adhere to collagen type IV, an extracellular matrix component of basement membranes. Similarly, multilineage colony-forming unit (CFU-MIX) progenitors from CML bone marrow do not adhere to fibronectin or its adhesion promoting fragments but adhere to collagen type IV. Unlike committed progenitors from normal bone marrow, CML single-lineage burst-forming units-erythroid and granulocyte/macrophage colony-forming units fail to adhere to fibronectin or its components but do adhere to both collagen type IV and laminin. Evaluation of adhesion receptor expression demonstrates that fibronectin receptors (alpha 4, alpha 5, and beta 1) are equally present on progenitors from normal and CML bone marrow. However, a fraction of CML progenitors express alpha 2 and alpha 6 receptors, associated with laminin and collagens, whereas these receptors are absent from normal progenitors. These observations indicate that the premature release of malignant Ph1-positive progenitors into the circulation may be caused by loss of adhesive interactions with stroma and/or fibronectin and acquisition of adhesive interactions with basement membrane components. Further study of the altered function of cell-surface adhesion receptors characteristic of the malignant clone in CML may lead to a better understanding of the mechanisms underlying both abnormal expansion and abnormal circulation of malignant progenitors in CML.