It is demonstrated that normal red cells are capable of reacting with antiglobulin (Coombs) serum so as to reduce its power to agglutinate Rh sensitized cells. The reaction is shown to be a specific combination between antiglobulin antibody and a globulin-like antigen on the normal cell surface. This antigen may be modified by the action of trypsin in such a way as to render it non-reactive, but the reactivity of the cell may be restored by treatment with normal human serum.