Sperm DNA is transcriptionally inactive; therefore, sperm functions are regulated by proteins present in mature sperm, with potential for these proteins to be a fertility marker. A testis-specific isozyme of Angiotensin Converting Enzyme (tACE) is involved in regulation of key steps leading to fertilization in rodents and humans. The overall aim of this thesis was to investigate the role of tACE in bull fertility. Using a custom anti-tACE immunoserum, tACE was detected in post-meiotic germ cells in the testes and localized to the acrosomal region of sperm head and principal piece of sperm tail. The extracellular portion of tACE, which contains the active site, was released during capacitation and was integral to its successful completion. In this study, tACE was tyrosine phosphorylated in a capacitation-dependent manner; perhaps this post-translational modification is involved in tACE shedding. Furthermore, cryopreservation and thawing decreased the content and activity of tACE in bull sperm. In fresh samples, this protein was in the acrosomal region of the sperm head, whereas after freezing and thawing, it re-localized from the acrosomal region to the postacrosomal region (equatorial segment and acrosomal ridge). Finally, regarding relationships among content, activity and localization of tACE and their association with field fertility in bulls, content and activity of this enzyme were significantly higher in high- versus low-fertility bulls. In summary, tACE content was selected as a single predictor of fertility and a link was established between tACE content and fertility.