NB/70K, a tumor-associated antigen of human ovarian epithelial tumor Fraction OCA, has been purified and identified as a glycoprotein which is stable in 0.6 M perchloric acid, binds to concanavalin A, and migrates electrophoretically with alpha-like mobility in barbital-buffered agarose at pH 8.6. NB/70K does not appear to contain normal serum, normal ovary, normal lung, or carcinoembryonic antigen-like cross-reacting antigenic determinants as measured by radioimmunoassay. NB/70K has been purified from ovarian antigen Fraction OCA by chromatography on gamma-globulin coupled to Sepharose 4B and by elution from acrylamide gels. NB/70K migrates as a single band with an apparent molecular weight of 70,000 in sodium dodecyl sulfate:acrylamide gel electrophoresis. A rabbit antibody raised against NB/70K was able to precipitate a polypeptide with a molecular weight of 70,000 as visualized by autoradiography of sodium dodecyl sulfate:acrylamide gels. A radioimmunoassay has been developed for measuring NB/70K activity, using Staphylococcus aureus protein A as a precipitating agent.