Prostasomes are membranous vesicles (150-200 nm diameter) present in human semen. They are secreted by the prostate and contain large amounts of cholesterol, sphingomyelin and Ca2+. In addition, some of their proteins are enzymes. Prostasomes enhance the motility of ejaculated spermatozoa and are involved in a number of additional biological functions. The possibility that they may fuse to sperm has never been proved. In this work, we studied the fusion of sperm to prostasomes by using various methods (relief of octadecyl Rhodamine B fluorescence self-quenching, fluorescence microscopy and flow cytometry) and we found that it occurs at acidic pH (4-5), but not at pH 7.5 pH-dependent fusion relies on the integrity of one or more proteins and is different from the Ca2+-stimulated fusion between rat liver liposomes and spermatozoa that does not require any protein and occurs at neutral pH. We think that the H+-dependent fusion of prostasomes to sperm may have physiological importance by modifying the lipid and protein pattern of sperm membranes.