Over the past 65 yr egg-yolk and milk have been routinely used in both liquid semen extenders and those used to cryopreserve sperm. However, the mechanism by which egg yolk and milk protect sperm during liquid storage or from freezing damage is poorly understood. Seminal plasma from many mammalian ungulates contains a family of proteins called Binder of SPerm (BSP). These proteins are secretory products of seminal vesicles that bind to sperm at ejaculation and modify the sperm membrane by removing cholesterol and phospholipids, which may adversely affect the ability of sperm to be preserved. Interestingly, BSP proteins bind to the low-density lipoproteins (LDL) present in egg yolk extender. The binding is rapid, specific and stable even after freeze-thawing of semen. LDL has a very high capacity for BSP protein binding. Furthermore, LDL prevents lipid efflux from the sperm membrane and maintains sperm motility during sperm storage. Recent, studies also indicate that casein micelles, α-lactalbumin and β-lactoglobulin, the major proteins in milk extenders also interact with BSP proteins present in the seminal plasma. Additionally, casein micelles reduced lipid loss from the sperm membrane and maintained sperm function during storage. These data indicate that the interaction between BSP proteins of seminal plasma and LDL or milk proteins could be the basis sperm protection.