The leukocyte function-associated molecule 1 (LFA-1, CD11a/CD18) is a membrane glycoprotein which functions in cell-cell adhesion by heterophilic interaction with intercellular adhesion molecule 1 (ICAM-1). LFA-1 consists of an alpha subunit (Mr = 180,000) and a beta subunit (Mr = 95,000). We report the molecular biology and protein sequence of the alpha subunit. Overlapping cDNAs containing 5,139 nucleotides were isolated using an oligonucleotide specified by tryptic peptide sequence. The mRNA of 5.5 kb is expressed in lymphoid and myeloid cells but not in a bladder carcinoma cell line. The protein has a 1,063-amino acid extracellular domain, a 29-amino acid transmembrane region, and a 53-amino acid cytoplasmic tail. The extracellular domain contains seven repeats. Repeats V-VII are in tandem and contain putative divalent cation binding sites. LFA-1 has significant homology to the members of the integrin superfamily, having 36% identity with the Mac-1 and p150,95 alpha subunits and 28% identity with other integrin alpha subunits. An insertion of approximately 200 amino acids is present in the NH2-terminal region of LFA-1. This "inserted/interactive" or I domain is also present in the p150,95 and Mac-1 alpha subunits but is absent from other integrin alpha subunits sequenced to date. The I domain has striking homology to three repeats in human von Willebrand factor, two repeats in chicken cartilage matrix protein, and a region of complement factor B. These structural features indicate a bipartite evolution from the integrin family and from an I domain family. These features may also correspond to relevant functional domains.