ABSTRACT: The antigenic spectra of estrual or cystic follicular fluid (FF) consisted of 11 blood serum antigens while that of estrual or cystic ovary consisted of 9 identical antigens of which 2 were ovarian specific. The two ovarian-specific antigens were detected with antisera to estrual or cystic ovary but not with antisera to nonestrual ovaries. They were in higher concentration in estrual ovary than cystic ovary, were localized in bovine oocyte zona pellucida by immunofluorescence and light scattering, and cross-reacted with prostate tissue. Cystic FF contained a higher concentration (P < 0.05) of IgG than at any stage of the cycle except estrus, and a lower concentration (P < 0.05) of estradiol-17β than at estrus. It was hypothesized that increased levels of estradiol during estrus alter the permeability of the follicle wall unmasking a zona pellucida antigen in the preovulatory follicle. The appearance of this previously unexposed antigen at estrus could lead to an inflammatory-like response triggering ovulation.