The Kell blood group is a major antigenic system in human erythrocytes. Kell antigens reside on a 93-kDa membrane glycoprotein that is surface-exposed and associated with the underlying cytoskeleton. We isolated tryptic peptides and, based on the amino acid sequence of one of the peptides and by using the PCR, prepared a specific oligonucleotide to screen a lambda gt10 human bone-marrow cDNA library. Four clones were isolated, one containing cDNA with an open reading frame for an 83-kDa protein. All known Kell amino acid sequences were present in the deduced sequence; moreover, rabbit antibody to a 30-amino acid peptide, prepared from this sequence, reacted on an immunoblot with authentic Kell protein. The Kell cDNA sequence predicts a 732-amino acid protein. Hydropathy analysis indicates a single membrane-spanning region, suggesting that Kell protein is oriented with 47 of its N-terminal amino acids in the cell cytoplasm, and a 665-amino acid segment, which contains six possible N-glycosylation sites, is located extracellularly. Computer-based search showed that Kell has structural and sequence homology to a family of zinc metalloglycoproteins with neutral endopeptidase activity.