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A predictive test for determining whether motile populations of human spermatozoa will fertilize eggs in vitro has been an elusive goal of clinical research. We have developed an assay for the ability of motile human spermatozoa to bind fluorescein isothiocyanate-labelled mannosylated bovine serum albumin (Man-FITC-BSA) as a test for the presence of sperm surface receptors (lectins) for mannose ligands. Mannosylated ligands are present on the human zona pellucida and are involved in the species-specific binding of human spermatozoa to the zona pellucida. We now demonstrate in prospective blinded analysis that the fractional increase in acrosome loss following a mannose ligand challenge is highly correlated with the rate of fertilization in vitro. Using an incremental increase of acrosome exocytosis of >0.1 as a threshold to predict which specimens will yield normal fertilization, the assay has a sensitivity of 97.8%, a specificity of 83.3%, a positive predictive value of 95.7% and a negative predictive value of 90.7%. These data indicate that testing for a mannose-induced acrosome reaction may be useful in assessment of sperm function prior to in-vitro fertilization in order to assign males to conventional insemination or intracytoplasmic sperm injection protocols.