Seminal plasma can both inhibit and stimulate sperm function, making its use as a supportive medium somewhat contradictory. These effects are directed by the multifunctional action of numerous inorganic and organic components, but it is the direct association of seminal plasma proteins with the sperm membrane that is thought to exert the most significant response. In vitro handling of spermatozoa in preparation for artificial insemination may involve washing, dilution, cooling, freezing, re-warming and sex-sorting. These processes can alter proteins of the sperm surface and reduce seminal plasma in the sperm environment. This, among other factors, may destabilize the sperm membrane and reduce the fertilizable lifespan of spermatozoa. Such handling-induced damage may be prevented or reversed through supplementation of seminal plasma, but the effectiveness of this technique differs with species, and the source and subsequent treatment of both spermatozoa and seminal plasma. Seminal plasma appears to act as a protective medium during in vitro processing of ram spermatozoa, but this does not appear to be the case for bull spermatozoa. The reasons for this divergent effect will be discussed with particular emphasis on the influence of the major proteins of ruminant seminal plasma, known as BSP proteins. The biochemical and biophysical properties of these proteins are well documented, and this information has provided greater insight into the signalling pathways of capacitation and the protective action of extender components.