After neuraminidase treatment the electrophoretic mobility of mouse lymphocytes (B and T cells) and thymocytes was reduced to a similar value, .a diminution of about 40%, 60%, and 40%, respectively. Thus B and T cells and thymocytes may have a common surface charge after removal of sialic acid. The percentage of cells binding anti‐thymocyte serum was not changed; thus sialic acid does not seem to be an important part of the specific antigen of lymphoid cells. Similar amounts of sialic acid were released from B and T Ceils, whereas less than half of this was released from the thymocytes. Thus, whereas thymocytes are characterized by their low electrophoretic mobility and low sialic acid release and T cells by high electrophoretic mobility and high sialic acid release, B‐cells in contrast show a low electrophoretic mobility and high sialic acid release.