Large amounts of an odorant-binding protein have been isolated from submaxillary glands of mature male pig. This polypeptide molecule is sex-specific, being absent in females. On electrophoretic gels under denaturing conditions it migrated as a broad band with an apparent molecular mass of around 20 kDa. Electrospray mass spectrometry revealed the presence of three main components, whose mass differences are not interpretable as result of any common post-translational modifications, indicating the presence of distinct polypeptide chains. N-terminal Edman degradation yielded a single sequence of 29 amino acids. It includes the lipocalin signature (-G-X-W-) and shows clear homology with a subclass of odorant-binding proteins present in mouse saliva, nasal mucus and urine. The purified protein still retained small ligands tightly bound; among them 5alpha-androst-16-en-3-one and 5alpha-androst-16-en-3alpha-ol, both known sex pheromones for the pig, were identified. The protein also binds 2-isobutyl-3-methoxypyrazine, a good ligand for most odorant-binding proteins, with a dissociation constant of 5 microM.