The EOS47 antigen is a 100-kD cell surface glycoprotein selectively expressed by avian retrovirus-transformed eosinophils and their precursors. We have purified the EOS47 protein to homogeneity and used peptide sequence information to clone EOS47-encoding cDNAs. The open reading frames from these cDNAs predict a 738 amino acid protein with homology to human melanotransferrin, a membrane-found, transferrin-like protein that is expressed at high levels by a subset of melanomas, tumor cell lines, fetal intestine, and liver, but not by most normal adult tissues. The predicted protein sequence of EOS47 displays a 61% sequence identity with melanotransferrin and conservation of all 28 cysteine residues, indicating a similar tertiary structure. The finding that EOS47 lacks several of the iron-coordinating amino acids present in all transferrins suggests that it may be impaired in its ability to bind iron. In nonhematopoietic tissues, EOS47 is expressed at high levels by epithelial brush borders of small intestine and kidney and at lower levels by cells lining the sinusoids of the liver. Within hematopoietic tissues, EOS47 is restricted to a subpopulation of cells (1% to 5%) in bone marrow and early spleen and fluorescence-activated cell sorting of EOS47+ cells leads to a dramatic ( > 30-fold) enrichment of peroxidase+ eosinophils. In contrast, peripheral blood eosinophils are EOS47-, suggesting that the antigen is expressed by newly formed eosinophils and that expression ceases shortly before these cells emigrate from the bone marrow into the peripheral blood. Our results show that melanotransferrin is a stage-specific marker of eosinophils and should be useful for their isolation and further characterization.