Objective: To evaluate the contraceptive ability of a synthetic Bin1b peptide in vivo in the rat. Design: Basic research. Setting: University laboratory animal service center. Animal(s): A peptide-based immunization model was developed; rats were injected with the Bin1b specific peptide. Intervention(s): A synthetic peptide segment, MCRSGERKGDICSDP-conjugated with KLH (Bin1b), was used to immunize male wistar rats. Freund's complete adjuvant was used as a control. Main Outcome Measure(s): Anti-Bin1b levels in sera were evaluated by enzyme-linked immunosorbent assay (ELISA). Anti-Bin1b and control antisera were used to evaluate sperm function inhibition in vitro. The fertility of immunized rats was determined by mating experiment. The testis and epididymides were analyzed by histology. Result(s): Histological studies showed no evidence of orchitis or epididymitis in Bin1b-immunized animals. ELISA results revealed that the titers of anti-Bin1b antibodies in serum increased with the immunization process in immunized rats. Sperm recovered from the corpus epididymidis of the Bin1b-immunized animals exhibited a significant decrease in motility. Immunization of Bin1b also caused a reduction (25%) in fertility after the mating experiment. Conclusion(s): The present study has demonstrated that immunization with Bin1b peptide specifically interferes with sperm motility, resulting in a compromised fertilizing capacity of sperm. © 2010 American Society for Reproductive Medicine.