Results with modified human red cell membrane sialoglycoproteins indicate that alkali-labile sialic acid and amino groups are parts of the erythrocyte receptor sites recognized by common rabbit and human anti-M and -N sera. The „N“ antigen, demonstrable in MM glycoprotein preparations by rabbit anti-N, has structural properties which are similar to those of the MN receptors. Sialic acid, amino groups and carbohydate, susceptible to periodate oxidation, are not involved in the Ss antigen sites. The specificity of theVicia graminea lectin is dependent on free amino and carboxyl groups. Its affinity for the substances is increased by blocking of amino groups.