To detect antigenic molecules involved in immune complexes (ICs) in patients with epithelial ovarian cancer, we developed several monoclonal antibodies (Mabs) by hybridoma technology from mice immunized with ovarian cancer tissues. Hybridoma supernatants were differentially screened with a panel of ICs purified from ascites of patients with ovarian cancer and with ICs from pooled normal human sera by the enzyme-linked immunosorbent assay (ELISA). From about 6,000 supernatants screened in 6 fusions, 4 Mabs showing preferential binding to the ascitic ICs were selected. Their antibody specificity was further examined with serum-free conditioned media of various cancer cell lines by an ELISA inhibition assay. The reactivity of 3 Mabs was inhibited by the media of epithelial ovarian cancer cell lines, but not by any of the non-ovarian cell lines tested, suggesting that the target antigens were derived from ovarian cancer cells. Furthermore, ICs detected by one Mab, designated 2F11, were elevated in the sera from 18 of 42 (42.9%) patients with epithelial ovarian cancer, but not in those from 39 patients with benign ovarian tumors or from 29 healthy individuals (with the exception of 1 serum), using sandwich ELISA with protein A as the capture reagent.