With a seasonally polyestrus breeding structure, the unwanted domestic cat population has proven difficult to control. Various lethal methods have been used in an attempt to lower this population of cats. Recently, humane attempts to control "pest species," such as the feral cat, have focused on immunocontraception. SpayVacTM is a vaccine that uses antibodies raised against porcine (ZP) antigens to prevent fertilization of the ovum. SpayVac, delivered in a single dose, has been evaluated in fallow deer and several species of seals with >90% reduction in fertility and no adverse reactions. This study evaluated the effectiveness of SpayVac in reducing fertility in domestic kittens. Thirty female kittens were treated with SpayVac containing either Freund's complete adjuvant (FCA) or alum, or with a control vehicle. Kittens were monitored for side effects, estrus cycling at maturity, and fecundity. Anti-porcine ZP antibodies were quantified by ELISA. Immunohistochemical assays measured the species specificity of the antibodies produced and IgG binding in vivo. Despite high anti-porcine ZP antibody titers, neither formulation of SpayVac prevented estrus cycling at maturity or reduced fecundity. Immunohistochemical assays indicated that antibodies produced by cats treated with SpayVac recognized porcine ZP, but not feline ZP.