A full-length clone encoding a murine membrane glycoprotein, gp42, was selected from a mouse fibroblast cDNA expression library by screening with a polyclonal antiserum. The deduced amino acid (aa) sequence indicates that gp42 is a transmembrane protein of 273 aa with a large N-terminal portion exposed outside the cell and a short cytoplasmic domain. Computer assisted analysis shows that gp42 is distinct from previously characterized proteins, but shares a number of structural features with the class II histocompatibility antigens. The sizes of the extracellular domains of gp42 and of class II histocompatibility antigens are similar, the position of four cysteines and the location of several aa residues are conserved. Some of these conserved residues are also present in immunoglobulins (Ig) and in the neural-cell adhesion molecule, thus indicating that gp42 is a new member of the Ig superfamily.