Carbamylation of epsilon-amino groups of lysine of human blood group MM glycoprotein, some of its precursors and the blood group A B antigens gave products with 41% - 91% epsilon-amino group substitution. Even the most extensive carbamylation led to only marginal changes in the circular dichroic (CD) spectra of these substances and none in sedimentation coefficients studied. Nevertheless, carbamylation resulted in either increased or unchanged or decreased inhibitory activity of all blood group antigens tested depending solely on the source of the hemagglutinin used. Carbamylation of epsilon-amino groups of these blood group glycoproteins therefore leads to minor conformational changes, not involved with the primary blood group specificity, which is recognized by a large proportion but not by all corresponding antibodies and lectins.