A simple method avoiding the use of nonionogenic detergent has been developed for the isolation of two forms of boar acrosin from ejaculated sperm. Each of the two forms was electrophoretically homogeneous and showed one N-terminal sequence only; Val-Val. The two acrosin forms isolated differ in molecular mass and amino acid composition. alpha-Acrosin, the form showing a higher molecular mass (Mr approximately 50 000), is stable in acid media. When exposed to pH above 4 it is converted into beta-acrosin (Mr approximately 35 000) by autodigestion. The isolation of the alpha-form was therefore carried out below pH 4 to eliminate autodigestion to the beta-form. The beta-form is stable for a certain period at neutral pH, especially if stabilized by Ca2+ ions. The autodigestion of alpha-acrosin to beta-acrosin probably results in the liberation of the C-terminal portion of the molecule of the alpha-form; this portion is composed of roughly 85 residues and is rich in proline.