Extracellular proteinases and proteinase inhibitors have been known to play many important roles in various biological systems. They are also important for reproductive functions of the mammals. Kallikrein stimulates human sperm motility. Porcine antithrombin III (AT III) has been identified to be a sperm-motility-stimulating protein and a chemoattractant for boar sperm. AT III-immunoreactive molecules are also detected on the zona pellucida of ovulated porcine eggs and are suggested to act as a sperm receptor. Proteinase inhibitors in the seminal plasma act as decapacitation factors. Both the proteinase inhibitors in the seminal plasma and the trypsin-like protein(s) on the surface of the sperm head play important roles in the sperm-egg interaction. The trypsin-like protein(s) are also involved in the induction of acrosome reaction. Plasminogen activator was demonstrated to be a trigger of the zona reaction. In summary, the roles of extracellular proteinases and proteinase inhibitors in the mammalian fertilization process are diversified and worthy of further investigation.