Two IgG-type cold auto-agglutinins causing transiently occurring haemolytic anaemia in children are described. The antibodies were limited to the IgG fraction obtained by DEAE cellulose chromatography and were resistant to 2-mercaptoethanol treatment. One auto-agglutinin reacted ‘incompletely’ (in the en-zyme test), the other reacted ‘completely’ and its reactivity was furthermore abolished by treatment of red cells with proteolytic enzymes, but not with neuraminidase. Because of the inactivation of the corresponding antigen on red cells by proteolytic enzymes, this receptor belongs to the Pr complex, but it can be distinguished from Pr(1)/Pr(2), through its resistance to neuraminidase. Therefore a new symbol ‘Pr(a)’ is proposed. Based on these findings an immunochemical classification for antigens corresponding to cold auto-antibodies is given and a new nomenclature of the antigens which are inactivated by proteases is proposed depending on their determination or nondetermination by neuraminic acid.