Two partially purified proacrosin forms have been obtained from acid extracts of ram ejaculated spermatozoa by Sephadex G-100 column chromatography in 0.001 M HCl. The form obtained from the extracts of whole ejaculates with apparently higher molecular mass was free of any acrosin inhibitor and was used for autoactivation studies. The autoactivation followed a classical S-shaped activation curve and was a two-step process. The presence of calcium ions slowed down the autoactivation process and stabilized the active acrosin formed. The activity of latter rapidly decreased when the pH of the fully activated mixture was adjusted to pH 3. The activity was not restored by re-adjusting the pH to 8. A proacrosin with apparently lower molecular mass was isolated from the extract of washed spermatozoa. Both proacrosins, however, showed the same molecular mass Mr approximately 58 000 daltons, when examined by gel filtration in 0.1 M NaCl (pH 3).