Abstract: The present invention relates to acidic (pI 4.0) 86 kDA isoforms of a novel, polymorphic, testis-specific protein designated calcium binding protein 86 (CBP86). This protein is tyrosine phosphorylated during in vitro capacitation and bound calcium45 on 2-D gels, the latter effect abolished by dephosphorylation with alkaline phosphatase. CBP86 localizes to the principal piece of the human sperm flagellum in association with the fibrous sheath and is the first demonstration of a sperm protein that both oligomerizes and gains calcium binding capacity in a tyrosine phosphorylation dependent manner during capacitation.