Monoclonal cold agglutinins have usually been found t o be IgM(K) and t o be directed against the I/i antigens.'Three IgA(K) cold agglutinins were tested for antibody specificity. Based on the following criteria, they were shown t o be directed against the Pr, antigen of the Pr, /Prz/Pra antigen system. The antigens reacting with the IgA(K) cold agglutinins were inactivated by protease and treatment of human red cells with neuraminidase and were demonstrable on human erythrocyte glycoproteins. The terminal sugar of the determinant group of these antigens is N-acetylneuraminic acid (NANA), which is not involved in I/i, but also in Pr2 antigenicity. The antigen activity of these antigens and the Pr2 and I antigen activities were distributed t o different glycopeptide fractions obtained from red cell glycoproteins by ficin cleavage and Sephadex G-50 separation. Periodate oxidation of red cell glycoproteins, which causes a shortening of the polyhydroxy side chain at C-6 of C-9 NANA to a C-7 and/or C-8 NANA derivative, resulted in inactivation of these antigens, while the I antigen remained unchanged and the Pr, antigen was increased 16-64-fold. Dog red cells gave no or diminished reactions with the three IgA(K) cold agglutinins, in contrast to increased reaction with antiPrz. Based on the different reactions of the IgA(K) cold agglutinins with dog red cells, a Pr, heterogeneity Prlh/Prld was demonstrated.In spite of the determination of the Prlh/Prld/Prz and the MN antigens by NANA, both antigen systems were shown t o be unrelated. Blocking of free c-lysine amino groups by acetylation or blocking of the NANA carboxyl groups by amidation of red cell glycoproteins resulted in MN inactivation, while the antigens reacting with the IgA(K) cold agglutinins (like Pr2) remained unaffected.The anti-Prl specificity of IgA cold agglutinins and the predominance of anti-I/-i specificity of IgM cold agglutinins was discussed with respect t o interrelations between immunoglobulin classes and antibody specificities of cold agglutinins.As has been shown by Roelcke [26], the antigens Pr, and Pr, are also inactivated by neuraminidase (RDE), in contrast t o the Pr, antigen. Pr1/Pr2/Pra are present on all adult and newborn erythrocytes in equal amounts, which is not true for the l/i antigens [23].Two of the three Pr antibodies described by Roelcke [ 17, 261 were not of the IgM class. Anti-Pr, was a monoclonal IgA(K) cold agglutinin [ 141, anti-Prz was a monoclonal K-type macroglobulin. Anti-Pr, was a transiently occurring 1gG cold agglutinin (found in a child with Mycoplasma pneumoniae infection) [ 171.