In the present study, we identified cDNA clones of mouse acrosin from a testis lambda gt11 library. The deduced amino acid sequence indicates that mouse acrosin is initially synthesized as a single-chain polypeptide with a 16-residue signal peptide followed by a 23-residue light chain and then a 394-residue heavy chain; mouse acrosin zymogen contains 417 amino acid residues with a calculated molecular mass of 46,993 Da. The cDNA-derived sequence of mouse proacrosin shows a high degree of similarity with human and porcine proacrosins and major portions of bovine trypsin, including the active site residues, the recognition site for substrate, the location of 12 cysteine residues, and two potential N-glycosylation sites. The sequence homology suggests that mouse proacrosin is converted to a mature acrosin, which consists of the light and heavy chains with a combined molecular mass of 35,587 Da, by cleavage of the peptide bond between Arg23 and IIe24, and sequential removal of 23-, 26-, and 50-residue COOH-terminal segments. Using Northern blot analysis of RNAs from various mouse tissues, the acrosin gene transcript was present only in testis. The 1,800-base acrosin message was first detectable in 18-day-old testis. At the same time of testicular development, some of the acrosin mRNA was actually associated with polysomes. Also, in situ hybridization analysis suggests that the acrosin gene is expressed only in the round spermatid. Therefore, it is most likely that transcription of the mouse acrosin gene and subsequent translation of its mRNA first occur in the early stages of the round spermatid, and that the acrosin message is not under translational control.