Rat natural killer cell Met-ase-1 (RNK-Met-1) is a 30,000 M® serine protease (granzyme) found in the cytolytic granules of CD3- large granular lymphocytes (LGL) with natural killer (NK) activity. To characterize the genomic sequences responsible for the CD3- LGL-restricted expression of this gene, we screened a rat genomic library with RNK-Met-1 cDNA, and obtained bacteriophage clones that contained the RNK-Met-1 gene. The RNK-Met-1 gene comprises 5 exons and spans approximately 5.2 kilobases (kb), exhibiting a similar structural organization to a class of CTL-serine proteases with protease catalytic residues encoded near the borders of exons 2, 3, and 5. The 5'-flanking region of the RNK-Met-1 gene contains a number of putative promoter and enhancer regulatory elements and shares several regions of homology with the 5'-flanking region of the mouse perforin gene. We have prepared nested deletions from approximately 3.3 kb of the 5'-flanking region of the RNK-Met-1 gene, and inserted these upstream of the chloramphenicol acetyltransferase (CAT) reporter gene. These 5'-flanking RNK-Met-1-CAT constructs were transiently transfected into rat LGL leukemia, T-lymphoma, and basophilic leukemia cell lines. The transcriptional activity of the RNK-Met-1 5'-flanking region was strong, restricted to the RNK-16 LGL leukemia and controlled by several positive cis-acting regions spread over at least 3.3 kb. The longest and most active 5'-flanking region (-3341 to -33) was also used to drive specific expression of beta-galactosidase in RNK-16. These data are consistent with the NK cell-specific expression of RNK-Met-1 and suggest the potential utility of this gene promoter in the development of transgene models of NK cell biology in vivo.