Objective: To explore the contraceptive potential of the CatSper1 transmembrane domains and pore region in vitro. Design: In vitro study with human sperm and mouse fertilization. Setting: Andrology laboratory of an academic research center. Patient(s) and Animal(s): Normozoospermia and viripotent BALB/c mice. Intervention(s): The specific binding of an anti-CatSper1 IgG antibody (H-300) to CatSper1 was confirmed by Western blot and immunofluorescence. Sperm from humans and mice were incubated with H-300. Main Outcome Measure(s): The effects of H-300 on human sperm progressive motility, abnormal acrosome, hyperactivated motility, and mouse in vitro fertilization rates were analyzed. Result(s): A significant decline in sperm progressive motility was observed after 1, 2, and 4 hours of incubation with H-300; the change was mainly ascribed to the decline of fast progressive motility. Significant inhibition of the hyperactivated motility was observed after 5 hours of incubation with H-300. The incubation of mouse sperm with H-300 before insemination reduced the in vitro fertilization rate to 28% of control levels (72% inhibition). Conclusion(s): CatSper1 may be a potential target for immunocontraception, and the antibody may be a tool to study the function of ion channels in sperm in which relatively fewer methods can be applied. © 2009 American Society for Reproductive Medicine.