The cDNA for the common \Mac-1 subunit (CD11b) of the mouse LFA-1/Mac-1/p150,95 group of leukocyte cell adhesion receptors, formally designated integrin \2, has been cloned and sequenced. Clones were isolated from cDNA libraries made from J774 macrophage and WEHI-3B myelomonocytic tumor cells which express this subunit as a component of the macrophage activation antigen 1 (Mac-1), also known as complement receptor type 3 (CR3). This subunit is expressed as a single, abundant mRNA species approximately 2.7 kilobase (kb) in size. The 2422 base pair (bp) cDNA sequence obtained codes for a 771 amino acid protein organized with leader, extracellular, transmembrane, and cytoplamic domains of 23, 680, 23, and 46 amino acids, respectively, yielding an 82700 mature protein of 747 amino acids. The mouse \Mac-1 subunit is highly similar to its human counterpart with an overall sequence identity of 81% and identical positioning of 5 out of 6 potential N-linked glycosylation sites, as well as 56 Cys residues that are organized in repeating motifs characteristic of integrin \ subunits. The most highly conserved regions are the transmembrane and cytoplasmic domains where only 4 out of 69 amino acids differ, indicating that the functions associated with this domain in Mac-1-mediated processes, such as iC3b-triggered phagocytosis, have been evolutionarily conserved.