CA 125 was determined in the sera of 94 female patients afflicted with various gynecological diseases. Benign ovarian tumors and gynecologic malignancies outside the ovaries had normal CA 125 values in most cases. Patients with ovarian carcinomas in advanced stage had high levels. 6 women, who had hepatic metastases from ovarian cancer, showed a so-called prozone effect in serum titration, that means, the CA 125 concentration at first rises to reach a peak at a dilution of 1:2 to maximally 1:20, and then reveal a normal reciprocal dilution behavior. Whether hepatic metastases as a possible source of marker production are responsible for this phenomenon should be examined in future studies.