The antigens reacting with cold-agglutinin antibodies and present in the red cell membranes of human red cells were found in the water-phase when the washed membranes were extracted with n-butanol. The presence of these antigens was demonstrated by agglutination inhibition, complement fixation, and antibody inhibition as determined by the C1 fixation and transfer test. Although antigen and antibody were not able to react at 37°C when the antigen was present in the intact cell, after solubilization, antigen and antibody reacted equally well at 37°C and at 0°C. The amount of anti-I inhibiting activity present in extracts from adult and cord cells was roughly the same, although the amount of antibody fixed to the intact cord cells was less with cord cells.

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