A library of cloned cDNA to male mouse submaxillary gland poly(A)-containing RNA was constructed in the plasmid pBR322. Inserts containing sequences estimated to be in the 1-5% abundance class were identified by hybridization to radiolabeled cDNA and examined by nucleotide sequence analysis. A sequence coding for a peptide with 57% homology to the only complete kallikrein sequence reported to date (from pig pancreas) was identified by a computer search program. This insert appears to code for the COOH-terminal 149 amino acids of a protein presumed therefore to be a serine protease. Comparison of the predicted amino acid sequence of this protein with analogous sequences in the three characterized members of the mouse submaxillary gland kallikrein arginyl esteropeptidase group of enzymes revealed extensive homology, although not complete identity. Thus, there are at least four members of this enzyme family expressed in the mouse submaxillary gland.