A fertilizing mamualian sperm penetrates the zona pellucida an extracellular matrix s urrounding oocyte, before fusing with the oocyte plasma membrane. As a consequence of the acrosome reaction of sperm, the acrosomal components are released, and interact with the zona pellucida Acrosin, an endoprotease with a trypsin-like cleavage specificity, is localized in the acrosomal matrix as an enzymatically inactive zymogen, proacrosin. Acrosin has long been believed to be involved in the limited proteolysis of the zona pellucida, enabling the sperm to penetrate this extracellular matrix and gain access to the oocyte plasma membrane. In addition to the protease activity, acrosin and its precursor possess the capacity to bind carbohydrate groups of zona pellucida glycoproteins as well as synthetic neoglycoproteins with L-fucose or D-mannose. Thus, acrosin seems to have two physiological functions: Iimited proteolysis of the glycoprotein matrix of the egg zona pellucida, and recognition and binding of the zona pellucida in the initial stages of fertilization. Here I summarize the current data regarding the structure and function of mammalian sperm acrosin.