Normal human sera contain antibodies which agglutinate red cells treated with trypsin and other agents. By absorption experiments and testing of eluates from agglutinated cells, each type of modified cells appeared to be agglutinated by one specific antibody. Among these agglutinins only the one active against trypsin treated cells was present in human cord sera; it did not cross‐react with the other types of modified cells. This antibody was also the only one to cause reversible agglutination with histidine acting as the reversing factor.