An auto‐anti‐Ena, which reacts with trypsin, but not with ficin‐treated red blood cells, and which can be totally inhibited with sialoglycoprotein (SGP) isolates from red blood cells, is described. From comparative studies on this antibody and on the four known examples of allo‐anti‐Ena, it is clear that the term “anti‐Ena” describes a heterogeneous group of related but not identical specificities. The specificities contained within the auto‐anti‐Enadescribed are different from those within any of the sera containing allo‐anti‐Ena. Several of the specificities that have been included under the blanket term, anti‐Ena, complex with the MN SGP of normal red blood cells, but recognize different por