A monoclonal immunoglobulin (OC 125) has been developed recently, which is directed against an ovarian antigen (CA 125) expressed in coelomic epithelium during embryonic development. An immunoassay for the antigen in serum has been developed using this antibody, OC 125. We have begun a prospective study of patients with documented ovarian carcinoma to determine if measurement of CA 125 in serum and peritoneal washings can aid in management of these patients by revealing occult residual tumor following treatment.