Defining the valid hemizona assay: accounting for binding variability within zonae pellucidae and within semen samples from fertile males**Supported in part by the Contraceptive Research and Development Program (CONRAD), Eastern Virginia Medical School, under a Cooperative Agreement (DPE-2044-A-00-6063-00) with the United States Agency for International Development (A.I.D.) Rosslyn, Virginia, and by the South African Medical Research Council, Republic of South Africa. The views expressed by the author do not necessarily reflect the views of A.I.D.