The murine monoclonal antibody OC 125 recognizes an epitope on a molecule called Cancer Antigen 125 (CA 125). The CA 125 antigen is expressed in amnion and its derivatives of fetal coelomic epithelia (such as Müllerian epithelia, peritoneum, pleura and pericardium) and in many adult tissues (such as the epithelium of fallopian tubes, endometrium, endocervix, pleura and peritoneum). The normal endometrium produces CA 125 and this production can contribute significantly to the level of circulating CA 125 at the time of menstruation. During peritoneal irritation (hyperstimulation, salpingitis, ruptured ectopic pregnancy, laparotomy) peritoneally derived CA 125 significantly contributes to circulating CA 125 concentrations, giving elevated CA 125 values. The use of the CA 125 serum assay as a single diagnostic tool is restricted by the fact that the antigen CA 125 is produced by normal epithelia (of peritoneum, endometrium and benign ovarian cysts) and not only by the ovarian cancer cell.