Ovarian tumor associated antigen was isolated from human serous cystadenocarcinomas of ovary and purified by using affinity chromatography columns coupled to anti-normal female serum and anti-normal ovarian tissue gamma globulin. Antiserum was prepared from New Zealand white rabbits. The antiserum was absorbed with normal female serum and extracts of normal ovarian tissue thoroughly. The absorbed antiserum was appraised by immunodiffusion, immunoelectrophoresis and counter-immunoelectrophoresis with intensifiers. The absorbed antiserum has a specific immune precipitin with extracts of ovarian tumor tissue. Counter-immunoelectrophoresis with intensifiers was used to detect the ovarian tumor associated antigen in the tissue and serum of various patients. Twenty-eight of 40 ovarian tumor extracts and 1 of 5 cervical carcinoma extracts showed the presence of the cancer antigen. 14/40 were positive in the sera of ovarian tumor patients and 6/10 positive in the sera of ovarian serous cystadenocarcinoma patients; 1/5 positive in the sera of cervical carcinoma patients and 2/10 positive in the sera of ovarian cysts patients. No positive result for the sera of normal females was noted. The specificity, sensitivity and accuracy of the method were 94.7%, 42.3% and 83.3%, respectively.