A previously unrecognized blood group antigen is described. The antigen was shown to be different from many rare blood group antigens, to be inherited as an autosomal dominant character and not to be closely linked to the established blood group systems nor to several other polymorphic markers of blood. The antigen is well developed at birth. About 5% of Finns have this antigen. In contrast only 1 donor in 502 Swiss blood donors and 2 unrelated donors in 395 Swedish blood donors were found to be positive for the antigen. The antigen is provisionally called Nea and the respective antibody anti-Nea. At least three Na(a+) blood units were given to a patient on different occasions before anti-Nea was detected in compatibility tests. Since the discovery of the first anti-Nea, three additional examples of this antibody have been identified in serum from multitransfused patients. Anti-Nea is capable of shortening the in vivo survival of transfused incompatible Ne(a+) cells.