A 7.1 kb mini-gene construct containing cloned DNA from the murine parotid secretory protein (PSP) gene with 6.2 kb of the promoter, has previously been shown to direct specific mRNA expression to the salivary glands in transgenic mice. However, the level of transgene expression in the parotid gland was only a few percent of the endogenous level. This indicated that elements necessary for high-level expression are still to be found. In this study, we have searched for such regulatory elements in additional flanking regions by using a 25 kb cloned Pspb fragment containing the complete structural gene, 11.4 kb of 5'-flanking sequence, and 2.5 kb 3'-flanking sequence as a transgene. To distinguish the expression of the transgene from that of the endogenous gene, we took advantage of an allelic difference, using an oligonucleotide that recognized the mRNA from Pspb and the transgene but not that from the other allele, Pspa. The expression of the transgene was examined in animals homozygous for Pspa. Three independent integrations all exhibited a level of parotid-gland-specific expression that corresponded to that of the endogenous gene. Thus, sequences responsible for this high-level PSP mRNA expression are situated within the genomic DNA of the transgene.