The ovarian carcinoma-associated antigen CA-125 was determined in 144 women with various gynecological tumors (ovarian, breast, uterine), by using the heterologous monoclonal antibody OC-125. In 90% of the malignant ovarian tumors positive CA-125 values were correct, whereas their negative values were correct in 88% of the benign ovarian lesions. In 90% of the 58 patients with malignant tumors, where it was possible to observe tumor marker concentrations as well as the simultaneous tumor behaviour, a good correlation was found. Therefore, CA-125 serum determinations seem to be useful in the preoperative assessment of the importance of palpable ovarian tumors, as a parameter for evaluating therapeutic measures, and for early detection of recurrences of ovarian tumors.