Lymphocyte differentiation is accompanied by changes in the expression of various glycoproteins at the cell surface 1–4. Whether changes in the structures of individual glycoproteins, as opposed to changes in then expression, also occur during this differentiation is not known. To answer this question, we have studied the structures of the Thy-1 glycoprotein isolated from mouse thymic (immature) and peripheral (mature) T lymphocytes. This glycoprotein was chosen for study because there is already evidence that the Thy-1 glycoprotein can carry different carbohydrate structures on the same polypeptide backbone5 and because it is also known that peripheral T lymphocytes have more sialic acid and more sialyl-transferase activity than do thymocytes6. Barclay et al.5 indeed showed that there were differences between the carbohydrate compositions of the Thy-1 antigen isolated from rat brain and thymus. We believe this to be the only clear example of tissue-specific differences in carbohydrate content of a given glycoprotein. In contrast to Barclay et al., we have studied cells of a common differentiation pathway. Here we show, using a two-dimensional electrophoresis technique7, that differentiation can involve the modification of carbohydrate structures of individual glycoproteins, as Thy-1 molecules on peripheral T cells carry more sialic acid than do thymic Thy-1 molecules. We also show that there is a charge difference (which does not involve sialic acid) between the two allelic forms, Thy-1.1 and Thy-1.2, of the antigen.