We had previously reported the purification and partial characterisation of four distinct odorant-binding proteins from male mouse nasal epithelium. One of these, named OBP-I appeared to be a heterodimer, whose subunits, Ia and Ib showed significant similarity in their N-terminal amino acid sequences with hamster aphrodisin. In this paper, we report the complete amino acid sequences of these two polypeptide chains, as deduced from nucleotide sequences of their relative cDNA. These data confirm the high similarity of both proteins with hamster aphrodisin. A comparison with the sequences of other known OBPs indicate that they are more closely related to members of class I, including bovine OBP, rat OBP-I and pig OBP-I. A putative odorant-binding site is indicated by the presence of amino acid residues conserved with respect to the bovine protein, whose three-dimensional structure has been recently resolved. In-situ hybridisation has revealed identical expression patterns for the two proteins, further supporting the heterodimeric structure of these proteins in the nasal mucus.