F4/80 is a monoclonal antibody that recognizes a murine macrophage-restricted cell surface glycoprotein and has been extensively used to characterize macrophage populations in a wide range of immunological studies. Apart from the tightly regulated pattern of expression of the F4/80 antigen, little is known about its possible role in macrophage differentiation and function. We have sought to characterize the molecule at the molecular level, through the isolation of cDNA clones, and now describe the sequence of the F4/80 protein. The primary amino acid sequence demonstrates homology to two protein superfamilies. The NH2-terminal region consists of seven epidermal growth factor-like domains, separated by approximately 300 amino acids from a COOH-terminal region that shows homology to members of the seven transmembrane-spanning family of hormone receptors. The potential role of these distinct domains is discussed with respect to the possible function of the F4/80 molecule.