Mild as well as strong periodate oxidation of isolated erythrocyte N and M glycoproteins and glycopeptides gave extensive to complete destruction of N-specificities as measured with Vicia graminea extracts and of N- as well as M- activities determined with all but 1 of 8 animal anti-N and 13 anti-M sera. Results with human antisera differed somewhat, while the specificity of mildly oxidized N-glycoprotein was completely destroyed as determined with all 8 human anti-N sera used, that of strongly oxidized N-active substance was completely inactivated towards one of the human antisera; the remainder showed 63--94% destruction, and 2 sera indicated no effect of oxidation. Similarly, while 10 of 14 human anti-M indicated complete inactivation of M-specific glycoproteins and glycopeptides after mild or strong oxidation, 2 showed partial inactivation and 2 human anti-M sera showed no inactivating effect of oxidation. The most relevant findings of quantitative carbohydrate analysis of periodate oxidized N- and M-specific substance were extensive transformation of N-acetylneuraminic acid (NAN) to its C8 and C1 analogues on mild oxidation and pronounced destruction of NAN and its analogues on strong oxidation; however, some intact NAN always remained. In all instances galactose (Gal) was destroyed to a much larger extent in N-derived than in M-derived glycoproteins and glycopeptides.