Over the past few years, soluble forms of adhesion molecules and cell-surface proteins in general have become widely used tools, not only in the study of protein-protein interactions, but also as affinity probes to identify novel ligands for a given cell-surface molecule. Particularly useful in this respect have been soluble forms of proteins created by fusing the extracellular domains of the protein of interest to the Fc part of an antibody molecule (1). The resulting dimeric proteins combine many of the functional characteristics of the adhesion molecule of interest with properties of the Fc part of immunoglobulins, such as, e.g., its interaction with protein A or G. This technique has been exploited extensively, as for example for the identification and purification of ligands to all three known selectins, including the L-selectin ligands GlyCAM-1 and CD34, the E-selectin ligand ESL-1, and finally the P-selectin ligand PSGL-1 (2–6). In addition, such adhesion molecule-Ig chimeras have been useful in the functional characterization of known receptor-ligand interactions in a wide variety of experimental settings.