Antitumor immune response to colorectal cancer extracts was tested by leukocyte adherence inhibition (LAI) assay. Of 38 colorectal cancer patients, 26 (68.4 percent) were LAI positive. The sensitivity of LAI assay was found to be inversely related to the stage of the disease. In contrast, 2 of 50 (4 percent) healthy individuals, 2 of 37 (5.4 percent) patients with nonmalignant gastrointestinal diseases, and 2 of 32 (6.3 percent) patients with malignancies other than colorectal cancer were LAI positive. Serum carcinoembryonic antigen (CEA) and serum sialic acid (SA) also were determined in 38 colorectal cancer patients. Using LAI assay in combination with CEA determination could improve the detection rate of colorectal cancer.