Human ovarian follicular fluid (hFF) has angiogenic activity, although the causative factors are unknown. We recently found that hFF contains renin activity which converts renin substrate to angiotensin I (AI). Since the enzymatic cleavage product of AI, angiotensin II (AII), is a potent stimulator of new vessel formation, we have examined Sephadex G-25 column fractions of hFF and extracted hFF and plasma from individual patients for AII-like immunoreactivity (AII-IR). Eluent fractions from Sephadex G-25 column chromatography of hFF had significant AII-IR which eluted in the same fractions as synthetic AII. Individual, extracted FF samples contained approximately 10 times higher levels of AII-IR than extracts of plasma from the same patients. Serial dilutions of the Sephadex column fractions and extracted FF and plasma inhibited binding of 125I-AII to rabbit anti-AII serum in a manner parallel to the inhibition caused by synthetic AII, indicating that the detected immunoreactivity was not due to non-specific assay interference. In summary, the results indicate the presence of significant AII-like immunoreactivity in hFF. AII may now be considered as a potential mediator of the angiogenic activity present in hFF and may play an important paracrine and/or autocrine role in physiologic events in the ovary.