Abstract Substances with the human blood group Sd a character occur on red cells and are also present in secretions. In urine Sd a activity is associated with the Tamm and Horsfall (T-H) glycoprotein. About 4% of Caucasian individuals, whose red cells are Sd a negative, have a T-H glycoprotein that is without Sd a activity. The two immunologically distinct forms of T-H glycoprotein have almost identical qualitative and quantitative amino acid and carbohydrate compositions. The only exception is the N -acetylgalactosamine content which falls in the range of 1–2% for preparations from Sd(a+) individuals whereas the level is negligible in the Sd a inactive preparations. These results strongly indicate that N -acetylgalactosamine makes an important contribution to the Sd a determinant structure in the T-H glycoprotein.