This report describes the evaluation of a chemical test for T-antigen in rectal mucus as a screening test for colon cancer. The test, called the Mucus Strip Test, detects the disaccharide residue sialic acid-free beta-D-Gal(1-->3)-D-GalNAc or T-antigen, which accumulates in mucus from malignant cells and colonic mucosa adjacent to cancer but not in normal mucosa. Participants were an unselected case series of 660 persons undergoing colonoscopy, excluding those with ulcerative colitis, polyposis, Crohn's disease, or nonspecific inflammatory bowel disease. In the first study (n = 608) rectal mucus was collected after preparation of the bowel for colonoscopy; in the second study (n = 52) a modified protocol was used to collect mucus approximately 2 weeks before colonoscopy and again following preparation for the procedure. Mucus Strip Test results were compared to the diagnosis received after colonoscopy, which was classified as cancer, adenomatous polyp(s), and others (normal). Analyses were also stratified by previous history of large intestinal disease, classified as previous cancer; previous diagnosis of adenomatous polyp(s); or others. In the first study, T-antigen was detected in approximately 30% of mucus samples, and test results were independent of both diagnosis at colonoscopy and previous medical history. In the second study, T-antigen was detected in 85% of samples collected before and 96% of samples collected after preparation for colonoscopy, but test results were again independent of diagnosis and medical history.(ABSTRACT TRUNCATED AT 250 WORDS)