SummaryBlood group M and N active substances were demonstrated in human mammary gland tissue in both benign and malignant lesions. The precursor T (Thomsen-Friedenreich) antigen occurred only in cancerous tissue, where it was found regularly in the 15 gross cancers tested. The precursor Tn antigen also was found regularly in cancerous breast tissue. Both antigens were reactive in breasts with in situ carcinoma. The T antigen was not demonstrable in the 6 benign mammary glands studied; similarly, the Tn antigen was either absent or occurred only in traces. The T antigen was unmasked by sialidase treatment of healthy breast tissues. Anti-T antibody, present in all human sera, was severely depressed in many breast cancer patients compared to controls.