Capacitation requires removal of proteins secreted by the cauda epididymis. Previously, we isolated and cloned the HongrES1 gene from rat cauda epididymis and found that it was exclusively expressed there. Here we report that HongrES1 mRNA is also expressed in the guinea pig cauda epididymis using Northern blot analysis, and the molecular weight of its cognate protein is approximately 48 kDa by Western blot analysis. Therefore, we investigated whether HongrES1 was involved in regulation of sperm capacitation in guinea pig. The results show that HongrES1 antisera (HA) significantly enhances sperm capacitation with maximal stimulation at a dilution of 1:500. Capacitation was reversed when capacitated spermatozoa were re-exposed to HongrES1 protein (HP, 0.25 µg/ml). In other words, HP acted as a decapacitation factor. HA accelerated the onset of capacitation and promoted a sperm hyperactivated motility response. Sperm capacitation was accelerated by HA stimulation of extracellular calcium influx while HP prevented extracellular calcium from influxing. Indirect immunofluorescence staining finds HP localized over the acrosomal anterior region of the sperm head, which exfoliates gradually during capacitation incubation, and completely disappeared after the acrosome reaction. Thus, HongrES1 expressed by the cauda epididymis is a novel molecule that regulates the physiology of guinea pig sperm prior to fertilization. Mol. Reprod. Dev. 76: 984–993, 2009. © 2009 Wiley-Liss, Inc.