ABSTRACT: PSP-I/PSP-II heterodimer is a major protein of boar seminal plasma which is able to preserve, in vitro, the viability, motility, and mitochondrial activity of highly extended boar spermatozoa for at least 5 hours. However, little is known about the binding pattern of the heterodimer to the sperm plasma membrane and its eventual relation with the maintenance of the sperm functionality. The present study investigated the effect of exposing highly extended boar spermatozoa (1 million/mL) to 1.5 mg/mL of PSP-I/PSP-II for 0.5, 5, and 10 hours at 38°C on sperm characteristics and the changes in PSP-I/PSP-II localization as a result of both the addition of PSP-I/PSP-II to the extender and the incubation time. Exposure of the spermatozoa to PSP-I/PSP-II preserved sperm viability, motility, and mitochondrial activity when compared to nonexposed spermatozoa. This protective effect lasted for 10 hours (P < .05). After immunolabeling of highly extended semen with rabbit monospecific polyclonal antibody against PSP-I/PSP-II, the percentage of immunopositive spermatozoa declines over time from 71% (0.5 hours) to 49% (10 hours). However, more than 80% of spermatozoa remained labeled during the 10-hour incubation period if PSP-I/PSP-II was added. Scanning electron microscopy revealed 4 different binding patterns. The heterodimer was mainly localized to the acrosomal area, being redistributed to the postacrosomal area or lost during in vitro incubation. In conclusion, the protective effect of the heterodimer appears to be related to its adhesion to the acrosomal area, and the loss of this protective effect coincides with a stepwise redistribution of PSP-I/PSP-II during incubation.