A self-made radioimmunoassay CA125 kit (using OC125 monoclonal antibody ascites offered by Dr. Bast Laboratory which was purified, solidified and labelled with 125I) was used for serum determination in 80 patients with epithelial ovarian cancer and in 40 standard antigen samples. The results demonstrated a statistically significant correlation between our self-made CA125 kit and an imported CENTOCOR CA125 kit (P < 0.001). Clinical experience before operation in 40 patients with epithelial ovarian cancer, 34 with endometriosis and 39 with other malignant ovarian tumors suggested that the self-made CA125 kit was more sensitive in diagnosing epithelial ovarian cancer than the other two diseases. Post-operative follow-up in 86 non-recurrent cases and 101 recurrent cases showed a 96% positive predictive rate and 91% negative predictive rate. It is concluded that self-made CA125 kit is very useful in monitoring epithelial ovarian cancer.