We developed tests of sperm-oocyte interaction: sperm-zona binding, zona-induced acrosome reaction, sperm-zona penetration and sperm-oolemma binding, using oocytes which failed to fertilise in clinical in vitro fertilization(IVF). Although oocyte defects contribute to failure of sperm oocyte interaction, rarely are all oocytes from one wom-an affected. Low or zero fertilization in standard IVFwas usually caused by sperm abnormalities. Poor sperm-zona pel-lucida binding was frequently associated with failure of standard IVF and obvious defects of sperm motility or morpholo-gy. The size and shape of the acrosome is particularly important for sperm binding to the oocyte. The proportion ofacrosome intact sperm in the insemination medium was related to the IVF rote. Inducing the acrosome reaction with acalcium ionophore reduced sperm-zona binding. Blocking acrosome dispersal with an acrosin inhibitor prevented sperm-zona penetration. Sperm-zona penetration was even more highly related to IVF rates than was s We developed tests of sperm-oocyte interaction: sperm-zona binding, zona-induced acrosome reaction, sperm-zona penetration and sperm-oolemma binding, using oocytes which failed to fertilise in clinical in vitro fertilization (IVF). failure of sperm oocyte interaction, rarely are all oocytes from one wom-an affected. Low or zero fertilization in standard IVFwas usually caused by sperm abnormalities. Poor sperm-zona pel-lucida binding was frequently associated with failure of standard IVF and obvious defects of sperm motility or morpholo-gy. The size and shape of the acrosome is particularly important for sperm binding to the oocyte. The proportion of acrosome intact sperm in the insemination medium was related to the IVF rote. Inducing the acrosome reaction with acalcium ionophore reduced sperm- zona binding. Blocking acrosome dispersal with an acrosin inhibitor prevented sperm-zona penetration. Sperm-zona penetration was even more highly related to IV F rates than was s