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As a first step fertilization requires that sperm bind to the extracellular coat of the egg. It is generally accepted that binding is mediated by complex carbohydrates on the egg coat, recognized by carbohydrate-binding proteins on the sperm surface. In the mollusc bivalve Unio elongatulus, the main functional epitope of the carbohydrate ligand has been determined, and it shares several characteristics with other sugar structures involved in the fertilization process in different animal models. A polyclonal antibody against the Unio epitope reacts with the human ZP3 glycoprotein and the Unio ligand binds human spermatozoa in an in vitro assay. These findings are discussed in the light of the species specificity of sperm-egg binding at fertilization.Copyright © 2001 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]