The authors present their initial results with the hemizona assay (HZA), which was developed to predict the fertilizing potential of spermatozoa. The HZA uses the matching halves of a human zona pellucida from a nonfertilizable and nonliving oocyte, providing an internal control on zona-to-zona variability. Maximal binding of human sperm to the hemizona usually occurred after 4 to 5 hours of coincubation. Sperm from fertile men exhibited significantly higher binding capacity to hemizonae compared with sperm from men who had fertilization failure during in vitro fertilization (IVF) treatment. The HZA index is calculated as follows: (bound sperm from subfertile male) divided by (bound sperm from fertile male) X 100. These findings demonstrate that the HZA may be a useful diagnostic tool in male infertility evaluations.