CD6 is a cell surface receptor belonging to the scavenger receptor cysteine‐rich (SRCR) protein superfamily (SRCRSF). It specifically binds activated leukocyte cell adhesion molecule (ALCAM, CD166), a member of the immunoglobulin (Ig) superfamily (IgSF). CD166 was among the first molecules identified as a ligand for an SRCRSF receptor, and the CD6‐CD166 interaction was the first interaction characterized involving SRCRSF and IgSF proteins. We focus here on what has been learned about the specifics of the CD6‐CD166 interaction from in vitro analysis. The studies are thought to provide an instructive example for the analysis of interactions between single‐path transmembrane cell surface proteins. Using soluble recombinant forms, the extracellular binding domains of receptor and ligand have been identified and characterized in a variety of assay systems. Both CD6 and CD166 have been subjected to intense mutagenesis and monoclonal antibody (mAb) binding studies and residues critical for their interaction have been identified. The availability of structural prototypes of both superfamilies has made it possible to map the binding site in CD166 and, more recently, in CD6 and compare these regions to epitopes of mAbs that block, or do not block, the interaction. In addition, the molecular basis of observed cross‐species receptor‐ligand interactions could be rationalized. These studies illustrate the value of structural templates for the interpretation of sequence and mutagenesis analyses. Proteins 2000;40:420–428. © 2000 Wiley‐Liss, Inc.