Here, we compare the architecture of membrane receptors with extracellular Ig-like domains located within the leukocyte Ig-like receptor complex (LRC) of humans and mice. The receptors can be classified broadly into four groups, based on the homology of their Ig-like domains and gene architecture. Receptors in the first group are characterized by the presence of the Ig constant type 2-1 (IgC2-1) and variant Ig (vlg) domains, and include the leukocyte Ig-like receptors (LILRs) and murine paired Ig-activating receptors (PIRs). The second group of receptors possess an IgC2-2 domain and comprise the killer-cell Ig-like receptors (KIRs) and platelet collagen receptor glycoprotein VI (GPVI). The third group consists of receptors with IgC2-1, and IgC2-3 or IgC2-4 domains, and includes the receptor for IgA Fc (FCAR), NKp46 and murine Ly94. The fourth group, with a single extracellular IgC2-1 domain, consists of the leukocyte-associated Ig-like receptors (LAIRs). The genomic organization of and evolutionary associations between these receptors and their domains are examined.