The major fucose-binding protein of 53 kDa from boar spermatozoa was isolated to apparent homogeneity using a two-step procedure including high-performance gel filtration and reversed-phase chromatography. The N-terminal sequence of the protein revealed that it is identical with the sperm proteinase acrosin. By means of a solid-phase zona-binding assay based on the avidin-biotin system it was demonstrated that acrosin also interacts strongly with porcine zona pellucida. Thus, the acrosin molecule combines specific proteolytic activity with zona- and carbohydrate-affinity properties, i.e. previously unrecognized properties of a serine proteinase. It seems likely that this special affinity of acrosin directs the proteolytic activity to its structural target in the vivo situation.