Ascites fluid, chest effusion, cyst fluid, and serum mainly obtained from malignant-ovarian-cancer patients were applied to quantitate the concentration of the tumor marker CA125. The same procedure was also performed for ascites, cyst fluid, and serum mainly obtained from benign-ovarian-tumor patients. To know how CA125 proceeds from tumor cells into the circulation, a CA125-producing, ovarian-cancer-cell line (SHIN-3) was transplanted sub-cutaneously into nude mice. Then the relationship between the grown tumor size and CA125 concentration in serum was analyzed and the histo-pathological background was also evaluated. Quantitative analysis of the ascites and cyst fluid showed no remarkable difference between the malignant-tumor patients and benign ones: however, the CA125 concentration in serum of malignant-tumor patients was up to 90 times higher than in benign ones. The proceeding ratio of CA125 in its productive site (cyst fluid, for example) to that in the serum of patients with malignant lesions was 24 times as high as with benign lesions. No relationship between tumor sizes and the CA125 concentration in serum in nude mice was observed. And in the mice which had high CA125 in serum, the capsules around the tumor tended to be destroyed and vessel infiltration by tumor cells was remarkable.