The present study was conducted to assess the mechanism of in vitro interference with fimbrial ovum capture by the ovum capture inhibitor (OCI) which we have recently demonstrated in endometriosis peritoneal fluid (PF). A golden hamster oviduct exposed to either endometriosis or nonendometriosis PF for 20 minutes at 37 degrees C was examined by scanning electron microscopy. Exposure of the oviduct to endometriosis PF reduced fimbrial activity of ovum capture and developed an OCI-related membrane on the fimbria, by which fimbrial cilia were completely concealed. This was not the case for nonendometriosis PF. Subsequently, an oviduct having been exposed to endometriosis PF was retrogradely flushed, by which the OCI-related membrane was ballooned and removed. The flushed oviduct resumed its activity of ovum capture. The OCI-related membrane appeared a cause of OCI interference with fimbrial ovum capture by preventing the contact between the fimbrial cilia and the cumulus oophorus.