PIP: Researchers collected serum samples from 23 infertile patients with sperm immobilizing antibodies (SI-Ab) and 1 pregnant patient from the Department of Obstetrics and Gynecology at the Hyogo Medical College in Japan to screen sera to determine whether they contained factors to inhibit sperm-zona pellucida tight binding. They used the recently developed hemizona assay (HZA) to test for this binding. The HZA assay showed that all 23 serum samples inhibited sperm-zona pellucida tight binding. The hemizona index (HZI) ranged from 3-53 with a mean of 18.1 (standard deviation of = or - 12) compared to a normal (HZI) of 100. Serum samples with titers 10 of SI50 inhibited sperm-zona binding as well as those with titers -or= 10 of SI50 (HZIs=17.3 vs. 18.9; p.1). All 23 serum samples bound to the surface of sperm plasma membrane after 1 hour coincubation as evidenced by the fact that they all demonstrated 50% IgG beads bound. Further the results of the indirect immunobead test (I-IBT) showed that positive sera (+or= 20% IgG beads) significantly inhibited binding more than negative sera (20% IgG beads bound) (HZIs=12.4 vs. 24.4; p.05). Yet serum with positive I-IBT for IgM did not affect sperm-zona binding (HZIs=17.1 vs. 19.4; p.1). No association existed between HZI and site of IB binding. The researchers interpreted theses results to mean that sera with both SI-Ab and antibodies recognized I-IBT for IgG and IgA may play a significant role to inhibit the sperm-zona pellucida tight binding. In conclusion, physicians should expect patients with low HZI to have more problems conceiving than those with normal HZI. In vitro fertilization using heat inactivated human cord serum or donor serum may help them to conceive.