In mammals, seminal plasma is a complex mixture of secretions from the epididymis and from the various accessory sex glands. The anatomy of accessory glands, as well as their chemical composition and the functions of their secretions are varied among species. In addition, the molecular composition of seminal fluid is very complex. Seminal plasma plays an important role in the fertilizing ability of sperm and the succes of the cryopreservation process. Proteins from seminal fluid have been implicated in the regulation of acrosome reaction, in the regulation of Ca+2 uptake by sperm and in the modulation of sperm capacitation. Seminal plasma protein composition varies from species to species but in all the cases investigated so far these components have important effects on sperm function.