Proacrosin is the zymogen of acrosin, a serine protease localized in the acrosomal matrix of mammalian sperm. Proacrosin/acrosin binds to solubilized zona pellucida glycoproteins (ZPGs) and various polysulfates in a non-enzymatic mechanism. In addition, both polysulfates and ZPGs induce proacrosin activation once they bind to the polysulfate-binding domain (PSBD) of the enzyme. We show here that the peptide ⁴³IFMYHNNRRYHTCGGILL⁶⁰ inhibited the proacrosin activation induced by either fucoidan or ZPGs. In addition, the peptide was recognized by the monoclonal antibody C5F10, which is directed against the PSBD region. Our data suggest that the PSBD is composed of many “subsites” that may or may not interact with each other.