Serum levels of tumor antigens NB/7OK and CA 125 were determined in 101 samples by radioimmunoassays with the use of murine monoclonal antibodies. Among serum samples from ovarian cancer patients, elevated NB/7OK levels were found in 87% of samples that contained elevated CA 125 levels. With either assay, only 2% of patients with nonmalignant diseases had elevated antigen levels. No quantitative correlation was found, however, between levels of NB/7OK and CA 125. When antigen present in serum was purified by gel filtration on Sephacryl S-200, CA 125 eluted with an apparent molecular weight in excess of 100 kilodaltons whereas NB/7OK eluted with an apparent molecular weight of 70 kilodaltons. Thus the NB/7OK and CA 125 radioimmunoassays appear to measure two different populations of antigenic material.