Sialic acids are structurally and topographically well-suited to function as ligands in cellular recognition events. Sialoadhesin (Sn) is a sialic acid binding receptor uniquely expressed by macrophage subsets. It is a member of the immunoglobulin (Ig) superfamily with 17 extracellular domains. Sn is a prototypical member of the siglec family of sialic acid binding proteins, which includes CD22, myelin-associated glycoprotein, CD33, and siglec-5. These membrane proteins are involved in discrete functions in the hemopoietic, immune, and nervous systems. The sialic acid binding region of siglecs is localized within the membrane-distal, amino-terminal domain and in the case of Sn, it has been characterized in atomic detail by X-ray crystallography, nuclear magnetic resonance, and site-directed mutagenesis. Our studies on Sn indicate that this receptor is likely to function as a macrophage accessory molecule in a variety of cell-cell and cell-extracellular matrix interactions. CD33 and siglec-5 are also expressed on macrophage subsets as well as other myeloid cells. However, unlike Sn, the properties of these molecules indicate a predominant role in signaling functions rather than in cell-cell interactions.