Hypertonic extracts from human fetuses (10--22 wk of gestation) were used to test the sensitizaton of leukocytes from cancer patients against fetal antigens in a direct, microcapillary tube assay system. Leukocytes were simultaneously exposed to a panel of allogeneic tumor extracts and a panel of fetal extracts. Leukocytes from 24 gastric cancer patients, 43 colorectal cancer patients, and 13 lung cancer patients were assayed with extracts obtained from gastric, colorectal, and oat cell carcinomas, respectively, and these extracts were also used with leukocytes from 41 patients bearing tumors of various other organs. Significant migration inhibition by tumor extracts was observed in 81.6% of the tests with gastric cancer, 67.4% of the tests with colorectal cancer, 69.0% of the tests with lung cancer, and 51.2% of the tests with other types of cancer. With fetal extracts, significant migration inhibition occurred in 58.3, 58.7, 59.6, and 54.9% of the tests, respectively. Reactivity against fetal extracts did not depend on the gestation age of the fetuses used for extraction. The conclusion was reached that the leukocytes of most of the cancer patients were sensitized against substances contained in fetal extracts irrespective of the type of tumor of the leukocyte donor. The cross-reactivity pattern suggested that 3-M KCl extracts of whole human fetuses contained a complex mixture of specificities related to the various fetal organs and tissues, which may have represented counterparts to most of the tumor-associated specificities.