Clones of cDNA encoding two serine proteinases were isolated from a cDNA library prepared from rat duodenum mRNA. The deduced amino acid sequences consisted of 248 residues and possessed a high level of homology to one another and to the sequences of granzymes, cathepsin G, and mast cell proteases I and II. Analysis of the enzymes' primary structures allowed the identification of the catalytic amino acid triad and the prediction of the substrate specificity. Northern blotting experiments showed that while one of these proteinases is expressed only in duodenum, the other enzyme is present in duodenum, lung, and spleen. It is supposed that these proteinases may play an important role in the function of an organism's defence systems.