beta-Microseminoprotein is a small, nonglycosylated protein, rich in disulfide bonds, which is present in the secretions of the airways, the gastrointestinal tract, and the urogenital tract. Its function is unknown. It was originally characterized in the human and it has been difficult to identify the homologous protein in species other than primates. We have purified beta-microseminoprotein from rat prostate and from amino acid sequencing we have been able to clone the protein. The results reinforce conclusions reached earlier that beta-microseminoprotein is a rapidly evolving protein. Overall amino acid identities are only 45, 50, and 40% when the rat protein is compared with the proteins from the human, the ape, and the pig, respectively. However, the 10 cysteines are all completely invariant between these four species. The cloning of beta-microseminoprotein in the rat have substantially improved the possibilities to reveal the function of this mucosal surface protein.