SUMMARYHaemagglutination inhibition studies using isolated membrane components from rat red blood cells showed that the activity of the major antigenic system RT2 was contained exclusively in the glycoprotein fraction and confirmed immunochemically the existence of two alleles,RT2aandRT2b. The SDS‐PAGE analysis of the membrane showed two interconvertible glycoproteins of apparent molecular weight 72,000 and 36,000, and the higher molecular weight component appeared to be a dimer of the lower molecular weight one. Analysis of the various membrane preparations after125I‐labelling showed that the glycoproteins existed in at least two molecular species on the surface of the red cells and that one species was more readily radio‐iodinated and dimerized than the other. The RT2 antigenic specificities were preferentially associated with this comp