Summary. The lack of the very common red cell antigen Ena is a rare recessive character. En(a‐) cells are further unusual in having only about 33% of the normal amount of sialic acid and in having an electrophoretic mobility about 60% of normal. These abnormalities adequately explain other peculiarities of En(a‐) cells: their weak MN antigens, their ability to be agglutinated, though suspended in saline, by appropriate incomplete Rh antisera, and their preferential agglutination by certain seed extracts and non‐immune animal sera. Ena is antigenic in rabbit as well as in man: an attempt to stimulate in a rabbit an antibody specific for En(a‐) cells did not succeed. The locus responsible for Ena is shown to be genetically independent of the loci for ABO, MNSs, Rh, Duffy and haptoglobins and not to be X‐ or Y‐linked. En(a+) hetereozygous cells show some of the changes of En(a‐) cells, in a modified but recognizable way. The physicochemical background and the nature of the genetic defect in this unusual blood group system are discussed.

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