The present study was undertaken to test the parallel detectability of ovarian cancer antigen CA 125 and gastrointestinal cancer antigen CA 19-9 in the sera of patients with malignant ovarian tumors, benign ovarian tumors, endometrial cancers, cervical cancers, colorectal cancers, and trophoblastic tumors and in early 1st-trimester pregnant as well as in healthy nonpregnant controls. In all kinds of gynecologic and colorectal tumors raised concentrations of both antigens were found with the exception of malignant nonepithelial ovarian tumors where neither of the antigens showed positive reaction. The most positive cases were found in the group with epithelial ovarian cancers. Of the two antigens CA 125 was the more responsive. No positive cases were found with either of the antigens in nonpregnant healthy controls or in patients with benign ovarian tumors. The parallel determination of the two antigens gives us a better opportunity to recognize pelvic tumors and further may enable us to distinguish ovarian and colorectal tumors.