CA 125 antigen levels were measured in patients with ovarian cancer (54 cases) by the RIA method using a monoclonal antibody OC 125 and were examined as a marker for ovarian cancer. The upper normal limit of CA 125 of 35 U/ml was derived from the mean value (15.7 U/ml) + 2 SD (9.3 U/ml) of CA 125 in healthy controls. The mean value of CA 125 in patients with ovarian cancer (1160 +/- 1850 U/ml) was statistically (p less than 0.001) higher than those of healthy controls, benign ovarian tumors (28 +/- 20 U/ml) and cervical cancers (226 +/- 526 U/ml). Elevated CA 125 levels were also found in the early stages pregnancy and endometriosis, but these cases did not show such high CA 125 values as those of ovarian cancers. In addition, CA 125 levels were not affected by the menstrual cycle. Among ovarian malignancies, elevated CA 125 values were specifically demonstrated in serous cystadenocarcinoma (positivity 89%) and markedly low in mucinous cystadenocarcinoma (positivity 16%). No positive correlation of CA 125 values with clinical stage (FIGO) were found in any ovarian cancer patients. The rise or fall of CA 125 level was well correlated with the progression or regression observed in cancer patients with positive CA 125 levels. In conclusion, serum CA 125 determinations may be useful in patients with ovarian cancer (except for mucinous type) for diagnosis and for monitoring the results of treatment.