This study aimed: (1) to evaluate the zona-binding capacity of patients with abnormal sperm morphology, using standard hemizona assay (HZA) conditions and increasing sperm insemination concentration during the assay and (2) to determine the insemination concentration needed to obtain equality in the number of tightly bound sperm to matching hemizonae, using sperm from teratozoospermic patients versus proven fertile controls. The minimum concentration of motile sperm from fertile controls necessary to validate HZA results was 250,000/mL (35.4 +/- 5.6 tightly bound sperm; mean +/- SE). The "effective number of sperm" (morphologically normal with high motility) was 60,750/mL. Each teratozoospermic patient had a unique, (higher) sperm insemination concentration (range: 0.5 X 10(6) to 2.0 X 10(6) motile sperm/mL) necessary to equal the number of tightly bound sperm representing the lower 95% confidence interval for the control sample (at 0.5 X 10(6) motile sperm/mL) with the matching hemizona. These results suggest that the HZA may be used as an indicator of the sperm insemination concentration during in vitro fertilization in patients with teratozoospermia.