Spectrophotometric and gas‐liquid chromatographic analyses on the carbohydrate moiety of tryptic erythrocyte glycopeptides from persons with Tn‐syndrome reveal a selective lowering of the galactose and sialic acid content, the degree being dependent on the percentage of polyagglutinable cells. Alkaline borohydride specifically releases N‐acetylgalactosaminitol, and the amount is correlated to the percentage of pathological erythrocytes. It is concluded that the alkali‐labile carbohydrate chains of Tn‐polyagglutinable red cells solely consist of N‐acetylgalactosamine linked to serine or threonine. Experiments with heterophile agglutinins whose specificity is known are in line with the above‐mentioned results. As judged from SDS‐polyacrylamide gel electrophoresis the three major membrane glycoproteins are affected to a different extent by the defect.