Adult blood cells of subgroups A1 and A2 as well as cord blood cells of group A are agglutinated by native immune anti-A sera of human origin almost to the same extent, provided saline solution is used as the diluent. If saline solution is replaced as a diluent by normal adult serum of group A or AB, immune anti-A sera of human origin allow the differentiation of adult A1 and adult A2 cells. Under these conditions, cord blood cells of group A frequently resemble adult cells of subgroup A2 more than adult cells of subgroup A1. The addition of a solution of purified A substance to human immune anti-A sera results in the neutralization of the antibody content against all subgroups of A when saline solution is used as the diluent. Neutralized anti-A sera frequently agglutinate A1 cells up to the original titer, without agglutinating A2 cells at all, if saline solution is replaced as a diluent by normal adult serum. Cord blood cells of 34 babies belonging to the blood group A revealed wide variations of agglutinability when tested with this type of reagent, exhibiting differences corresponding to the subgroups of adult A bloods, though on a lower level of agglutinability. About half of the babies' cells reacted like adult A2 cells or even weaker. The other half could be placed between adult A2 and adult A1 cells, rarely reaching the agglutinability of adult A1 cells. Hirszfeld's theory of “pleiades” is discussed in its relation to the experimental results described in this paper.