Fertilization of eggs from all species studied to date is characterized by an increase in the internal concentration of calcium ions ([Ca2+]i) which is responsible for egg activation and initiation of development. In mammalian eggs, a distinctive characteristic is the presence of [Ca2+]i oscillations that exhibit species-specific frequency and persistence. We and others have provided experimental evidence that an unidentified spermatozoal [Ca2+]i oscillation-inducing factor (SF) may be responsible for triggering these increases. These findings support the theory that, after gamete fusion, the spermatozoon introduces into the egg a SF(s) responsible for the initiation of oscillations and embryo development. The active component of SF has not been fully characterized, but the recent discovery of a sperm-specific phospholipase (Zeta), suggest that this molecule may represent the active factor from the spermatozoon. Accordingly, we have investigated whether or not (Zeta) is present in stallion spermatozoa and whether the amount of (Zeta) is reduced in spermatozoa of subfertile animals.