Before implantation the blastocyst is maintained within a proteinaceous coat, the zona pellucida, which prevents polyspermy and ectopic pregnancy. An extracellular trypsin-like activity, which is necessary for hatching from the zona pellucida in vitro, is localized to the abembryonic pole of the blastocyst. Upon hatching, the extracellular matrix-degrading proteinases urokinase plasminogen activator (uPA) and matrix metalloproteinase 9 (MMP-9) are thought to promote blastocyst invasion. However, gene disruption experiments have demonstrated that uPA and MMP-9 are dispensable and, thus, that other key enzymes are involved in implantation. In this study, a novel implantation serine proteinase (ISP1) gene, which is distantly related to haematopoietic tryptases and represents a novel branch of the S1 proteinase family, was cloned. ISP1 is expressed throughout morulae and blastocysts during hatching and outgrowth. Abrogation of ISP1 mRNA accumulation using antisense oligodeoxynucleotides disrupts blastocyst hatching and outgrowth in vitro. The results of this study indicate that the ISP1 gene probably encodes the long sought after 'hatching enzyme' that is localized to the abembryonic pole during hatching in vitro. ISP1 is the earliest embryo-specific proteinase to be expressed in implantation and may play a critical role in connecting embryo hatching to the establishment of implantation competence at the abembryonic pole of the blastocyst.