THE union of sperm and egg is a special membrane fusion event that gives a signal¹ to begin development. We have hypothesized² that proteins mediating cell–cell fusion events resemble viral fusion proteins^3,4and have shown that PH-30, a sperm surface protein involved in sperm–egg fusion⁵, shares biochemical characteristics with viral fusion proteins6. We report here the complementary DNA and deduced amino-acid sequences of the mature αand β subunits of PH-30. Both are type-I integral membrane glyco-proteins. The α subunit contains a putative fusion peptide typical of viral fusion proteins and the β subunit contains a domain related to a family of soluble integrin ligands found in snake venoms. Thus, the PH-30α/ βcomplex resembles many viral fusion proteins in both its membrane topology and its predicted binding and fusion functions.