The free-ranging stray cat population has proven difficult to control in this country leading to mandatory neutering and cat licensing laws in some communities. In other places, various lethal methods, including hunting, trapping, and poisoning have been utilized in an attempt to lower this feral cat population. Due to limited long-term success and public opposition, these methods are not widely accepted. Trap, Test, Vaccinate, Alter, and Release (TTVAR) programs are becoming an increasingly popular alternative. Unfortunately, the cost and logistics of these non-lethal surgical sterilization programs have limited their success in lowering the total un-owned cat population. Recently, humane attempts to control “Pest species,” such as rabbits, have turned to a vaccine to provide immunocontraception. SpayVac™ is a single dose vaccine that prevents pregnancy by interfering with fertilization. It has already been shown to reduce fertility in rabbits, barbary sheep, and several species of seals with greater than 90% effectiveness. A similar vaccine was used successfully in mountain lions. This study will measure the effectiveness of SpayVac™ in reducing fertility in domestic cats. If successful, SpayVac™ would be an important tool in the effort to reduce the stray cat overpopulation humanely.