One of the recently described antigens broadly expressed on human leukocytes is CDw149, which was defined at the 6th Human Leukocyte Differentiation Antigen (HLDA) Workshop by means of 2 monoclonal antibodies (mAbs). Molecular characterization of this antigen has been lacking. In the present study we demonstrate that these anti-CDw149 mAbs actually recognize a clustered subset of a well-defined membrane protein, CD47, also known as integrin-associated protein (IAP). This clustered subset is present on leukocytes but not erythrocytes. The anti-CDw149 mAbs bind with only low affinity to a monomeric (unclustered) subset of CD47 but with high avidity to the CD47 clusters. A fraction of CD47 is associated with large complexes containing cytoplasmic signaling molecules (Src family kinases and heterotrimeric G-proteins) similar to glycosphingolipid-enriched microdomains (GEMs), which may explain the previously described signaling capacity of CD47. The low-affinity anti-CD47 mAbs may be useful tools targeting specific receptor complexes involved in cell activation. Specific reactivity of low-affinity mAbs with clustered subsets of cell surface antigens may more generally explain the nature of poorly defined "activation forms" or activation neoepitopes described previously for several cell surface molecules.