Measurement of CA125 is well established in the diagnosis and follow-up of ovarian cancer. CA125 determination is usually performed with the monoclonal antibodies OC125 and Mil. For the IMMULITE(R) OM-MA chemoluminsence assay (DPC Biermann, Bad Nauheim, Germany) the monoclonal antibody OV185 is used. We analysed CA125 serum concentrations of patients with benign and malignant ovarian tumors with the IMMULITE(R) OM-MA assay by defining a cut-off at 90% specificity level. Sera of 130 patients with benign and 119 patients with malignant ovarian tumors at different FIGO stages were analysed. By calculating a cut-off of 46 U/ml, in 87 ovarian cancer patients, elevated CA125 concentrations were detected (78% sensitivity). There were no significant differences between CA125 concentrations of tumors of different histological type. The measurement of CA125 by IMMULITE(R) OM-MA is a simple and sensitive procedure.