Landsteiner and Levine (1927) established that human erythrocytes possess type-specific biochemical differences in addition to the factors determinable by iso-agglutination. With the aid of sera obtained by immunizing rabbits with human erythrocytes, they succeeded in proving the presence of new, until then unknown, M and N antigens in the erythrocytes of man. Thus besides the differentiation into the O, A, B and AB groups a further subdivision into so-called types M, N and MN was introduced. The presence of the M- and N-antigens in the human erythrocytes could be established only with the aid of immune sera, since no natural isoantibodies anti-M and anti-N were found in the human organism. The finding of the M- and N-factors in the erythrocytes served as the starting point for the further search of these factors in other cells of the human tissues and organs.