We have searched for genetic variation in the expression of salivary androgen-binding protein (ABP) in a wide variety of mice and other rodents. ABP was present in the salivas of mice of all species and subspecies studied. Genetic studies have identified three common variants of the ABP Alpha subunit (Abpaa, Abpab, and Abpac) in Mus musculus populations with distributions that correspond roughly to those of the subspecies studied (domesticus, musculus, and castaneus, respectively). It appears that the ABP a and b polymorphisms conform to the hybrid zone between the domesticus and musculus subspecies characterized by others. Our studies suggest that the presence of Abpab in inbred strains may be due to a M. m. musculus contribution, perhaps via oriental fancy mice bred to European mice in the early lines leading to the common inbred strains. The relatively common occurrence of the ABP a type in other Mus species leads us to conclude that it is the ancestral type in mice. Further, the observation of what amounts to unique alleles in the three different subspecies indicates that microevolution of the protein has occurred. In a broader survey, ABP was also found in the salivas of Murid and Cricetid rodents generally. These findings suggest that ABP has an important functional role in rodent salivas.