There are two known alleles of the mouse parotid secretory protein (PSP) gene: Pspa and Pspb. Pspa is carried by DBA/2J mice and Pspb is carried by C57BL/6J mice. Eighty-eight mice derived from a F1(C57BL/6J x DBA/2J) to DBA/2J backcross were analysed for PSP mRNA expression in the sublingual glands. Expression was found in heterozygous mice only. This indicates that only Pspb is expressed in this tissue. Furthermore, it maps the allele-specific sublingual gland determinant within 3.4 cM of Psp. Previous analysis of Pspb identified an enhancer-like region in position -4.6 to -3.1 kb that was necessary for transgene expression in the sublingual glands. Here it is shown that the corresponding region in Pspa enhances transgene expression in the sublingual glands as efficiently. The implications for regulation of PSP mRNA expression in the sublingual glands are discussed.