Sera and follicular fluid from mares previously determined as having antibodies to the zona pellucida were exposed to porcine ovarian oocytes before insemination with boar spermatozoa in vitro. There was a dramatic decline in the number of spermatozoa bound to zonae compared to treatments with sera and follicular fluid from mares negative for zona antibodies. These data suggest that antibodies reactive with the zona pellucida may be responsible for reduced or even complete infertility in some mares. In a group of 50 randomly selected mares tested for antizona antibodies, 2 pregnant mares were found to be positive, one at 60 and the other at 90 days of gestation. The duration of zona antibodies was determined in 3 mares previously identified as zona positive. All were sampled at weekly intervals for at least 6 months. There was considerable variance in the presence of zona antibodies throughout the sampling period, suggesting that the presence of zona antibodies in mares does not necessarily correlate with their oestrous cycles. The periodic absence of detectable zona-reactive antibodies in zona positive mares suggests that multiple sampling may be required to eliminate the possibility of false negatives.