Sialoadhesin is a macrophage‐restricted adhesion molecule of 185 kDa that mediates sialic acid‐dependent binding to cells. It is expressed strongly by macrophages in lymphoid and haemopoietic tissues where it is likely to mediate cell‐cell interactions. Here we report the molecular cloning of murine sialoadhesin and show that it is a new member of the immunoglobulin (Ig) superfamily with 17 Ig‐like domains. COS cells transfected with a cDNA encoding full‐length sialoadhesin bound mouse bone marrow cells in a sialic acid‐dependent manner. Alternatively spliced cDNAs, predicting soluble forms of sialoadhesin containing the first three or 16 Ig‐like domains of sialoadhesin, were expressed in COS cells and the respective proteins purified. When immobilized on plastic, the 16‐domain form bound cells in a sialic acid‐dependent manner, suggesting that sialoadhesin can function in both secreted and membrane‐bound forms. The most similar proteins in the database were CD22, myelin‐associated glycoprotein, Schwann cell myelin protein and CD33. Like sialoadhesin, CD22 mediates sialic acid‐dependent cell adhesion. The sequence similarity of sialoadhesin to CD22 and related members of the Ig superfamily indicates the existence of a novel family of sialic acid binding proteins involved in cell‐cell interactions.