In present work the Authors examine the utility of CA 125 assay in patients affected by epithelial ovarian cancer. It is stressed that marker negativity does not necessarily imply the absence of disease. On the other hand the Authors underly the almost total absence of false positives, the only problem being with cases of benign gynaecological pathologies. The CA 125 assay appears to provide informations in the evaluation of response to therapy. Finally they note two more recent applications of monoclonal antibodies: first, marking these substances with radioisotopes; second, immunohistochemistry that shows the presence and location of the antigen at the histological level. (By editorial staff).